Simple Worked Example

In the following pages, we'll create a simple bar chart, using the visual, some data, an inbuilt template and Vega-Lite. Our finished visual is the bar chart on the left:

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Our Starting Data

From the sample workbook, we'll use a single column and measure. Here's how this looks in raw form:

Here, the [City] column represents a nominal category, and [Mean Temperature] is a simple measure that calculates the average temperature value for the given row context.

Think with Tables

Try to think about the dataset you wish to add to a Deneb visual in terms of how it will work as a table visual. This will help to understand the granularity of the dataset, and help to "think visually" when it comes to creating marks and layouts.

Adding the Visual

No Visual in your Report?

If you need assistance with this, refer to the Getting Started page.

Once you have added Deneb to your report, you should see it in the Visualizations pane, eg.:

You can then click the visual to add it to the report canvas, e.g.:

Adding the Data

We'll now add the [City] column and [Mean Temperature] measure to Deneb's Values data role, e.g.:

At this point, we need to create a specification, which is required to instruct Vega-Lite (or Vega) how your visualization should be rendered.

Opening the Visual Editor

We can now open the Visual Editor by selecting the visual header (...) and then Edit, e.g.:

This will display the Visual Editor in focus mode, and the Create New Specification dialog will be shown, e.g.:

Can't See the Dialog?

If you accidentally dismiss the dialog, or it doesn't show, you can bring it up by clicking on the New Specification button on the command bar, or using [Ctrl + Alt + N]

Selecting the Template

Using Vega or Vega-Lite requires a specification to be written using JSON. You can either select the [empty] template to begin editing with a bare-minimum specification with the dataset bound to it, or use a predefined template.

Let's choose the Simple Bar Chart template by clicking on it. Note that we now get some additional details about the template, as well as some required placeholders:

Assigning Placeholders

This template expects two placeholders: one for the the category axis and another for the measure axis. As we've already added our data, we can see these are already available in the dropdown lists, e.g.:

In our case, we'll add City to the Category placeholder.

About Data Types

You can choose either the column or the measure - the recommended data type is shown next to each one so that you can visually confirm the right type of field to bind to the placeholder. Using an different type might work, but be prepared to have to make some changes afterwards if you proceed with this.

Deneb can only see columns or measures added to the Values data role. This is a limitation of visuals in Power BI so that they cannot know more about your data model than they are supposed to. You can add or swap out a column or measure if it's not the one you want and the dropdown list will update accordingly.

Next, we'll click on the Measure placeholder and select the Mean Temperature measure:

Creating the Initial Specification

Now that all placeholders have been filled, the Create button is enabled:

Clicking on this will show the visual specification in the Visual Editor with our assigned placeholders, e.g.:

Reviewing Generated Specification

In our editor's Specification tab, we'll now have the following Vega-Lite JSON specification:

{
  "data": { "name": "dataset" },
  "mark": {
    "type": "bar",
    "tooltip": true
  },
  "encoding": {
    "y": {
      "field": "City",
      "type": "nominal"
    },
    "x": {
      "field": "Mean Temperature",
      "type": "quantitative"
    },
    "opacity": {
      "condition": {
        "test": {
          "field": "__selected__",
          "equal": "off"
        },
        "value": 0.3
      }
    }
  }
}

This breaks down as follows:

• There's a data object, which has a mapping of "dataset".
  • This is required for the specification, instructs that our visual's dataset should be used and should be left as-is.
  • Refer to the Dataset page for more details on how this works.
• We're using a bar mark for our data point and we're displaying a tooltip for this mark.
• The encoding for the visual maps our data as follows:
  • City is mapped to the y-axis and is a nominal/categorical field.
  • Mean Temperature is mapped to the x-axis and is a quantitative/linear field.
  • The opacity of our mark is linked to a special field called __selected__, which manages cross-filtering for us. This is explained further below, so just bear with us for now.

This is a simple single view specification, so once you're comfortable with the syntax, you should be able to move onto layered, concatenated or faceted views in no time 😉

We also have some other stuff going on in the editor's Config tab, thanks to our template...

Reviewing Generated Config

In our editor's Config tab, we'll have the following Vega-Lite JSON configuration. There's a lot going on here, and the intention is to provide you with a fleshed-out set of properties so that you can see how things can look with a fleshed-out set of properties. You don't need to worry too much about this for the moment.

{
  "autosize": {
    "type": "fit",
    "contains": "padding"
  }
}

Config is a nice feature of the Vega languages that provides us with the ability to style and set out the cosmetic elements of our visual design away from the main "logic" (which we have in the Specification tab). One thing that's handy for Power BI - due to the ability to dynamically size visuals - is automatic sizing for simpler (single or layered) views, so has been included in this template by default.

You can include your config in the specification's JSON at the top-level if you like, but the intention here is to separate the cosmetic aspects of the visual away from its logical ones, so we have a separate tab in the editor for this for those who wish to adopt this approach.

You can extend the config significantly to customize many parts of your visual, so it's worth diving into the documentation for this when you're ready.

Tooltips

Our template enables tooltips by default, so we can hover over a data point in our visual and this will display a default tooltip, e.g.:

If we were to set up a tooltip page that included one of the columns from our visual, then, the visual can resolve this, providing that you specify to use a report page tooltip instead of a default one, e.g.:

Context is Key

Note that report page tooltips will only work if your data point hasn't been transformed or mutated from the row context passed-into your visual's dataset. In the cases where data points cannot be reconciled back to the data model, the visual will display a default tooltip instead. Please refer to the Interactivity Features or Tooltips pages for further details.

Cross-Filtering

Many Power BI visuals allow you to cross-filter others by clicking (or Ctrl-clicking data points). We can also do this with Deneb, but there are elements of user experience that we need to consider: most Power BI visuals will dim or fade the un-selected data points so that our users understand what is happening between our visual and others.

This is normally opt-in, but our template already has a simple version of this set-up already. The relevant section of our specification is on lines 16 through 25:

{
  "opacity": {
    "condition": {
      "test": {
        "field": "__selected__",
        "equal": "off"
      },
      "value": 0.3
    },
    "value": 1
  }
}

Deneb uses a special field called __selected__ to track the status of each row of our dataset. We can use this in our encoding, along with a test to set the opacity value of our mark to 0.3 (or 30%) if it is not one of those that we've clicked (or Ctrl-clicked) on, e.g.:

Context Really is Key

Note that much like report page tooltips, Cross-Filtering will only work well if your data point hasn't been transformed or mutated from the row context passed-into your visual's dataset. In the cases where data points cannot be reconciled back to the data model, this may not work as intended. You also need to manage the visual effects of your selected and un-selected marks. Please refer to the Interactivity Features or Cross-Filtering (Selection) pages for further details.

Adding Further Config

The config that's present in the loaded template should already make the chart look quite at home in our report, but we'll make some small changes to the bar mark we're using. Update the bar property (on line 13) as follows:

  "bar": {
    "color": "#605E5C",
    "cornerRadiusTopRight": 10,
    "cornerRadiusBottomRight": 10
  },

We can apply our changes by clicking the Apply command, or pressing [ Ctrl + Enter ]:

Auto-Apply

Note that the Auto-Apply command can be used to ensure changes are applied as you type rather than manually, although this isn't recommended for complex specifications or ones that use a large number of data points.

Once we've applied the changes, our visual updates as follows:

Viewing the Finished Product

When you're finished, click on Back to report to bring you back out of focus mode and to the report canvas. Our visual will now be 'regular sized', thanks to the autosize configuration we applied earlier on:

![Returning to the report shows our visual specification "regular-sized".](./img/regular-sized-bar.png "Returning to the report shows our visual specification "regular-sized".")

Where Next?

We've used one of the inbuilt templates to produce a simple example, and add some integration features in the form of tooltips, and customized its appearance using config. A simple visual like this is just the tip of the iceberg and the way ahead is quite broad and expansive.

The purpose of this site is to provide detail on how to make the visual work for you within Power BI, rather than repeating the wealth of great documentation that's already out there. To learn more about some of the additional things the visual can offer (or understand some of its limitations) then please continue to read on or use the menu to explore further.

If you're hungry for more examples or further inspiration, then a good place to start is Vega-Lite's own gallery, or even Vega's gallery.