Month: May 2020

Dates and times are probably the most commonly used dimensions in Power BI charts, or any charts for that matter. Power BI contains a number of features that help to display data with respect to time. Features like the automated date hierarchy reduce the need for users to construct or connect to a date dimension table (even though they likely should), which helps casual users get to solution more quickly. This is particularly true of using date/time on the axis of a chart. There are a lot of options for displaying this data, and they may not all be that well understood. This article will attempt to explain a number of them.

The scenario

We will be working with thermostat data taken in 5 minute increments over several months. The shape of the data is relatively simple. There are measures for outdoor temperature and heating/cooling system run times in seconds, as well as a date/time dimensions names DateAndTime. An example can be seen below

We want to plot these runtimes over time, and we will be working with a “Line and clustered column chart” to do this. The 4 different heating/cooling runtimes are used for the column values, the Outdoor temperature is used for the line values (with average being the default aggregation behaviour). This gets us to our point – what is the best way to display time on the X axis?

Plotting with DateTime

When the DateAndTime column is added to the X axis, by default it is converted to a date hierarchy. This behaviour is on by default but can be turned off (and in many cases, should be). We initially want to work with the raw datetime value, so we can control that by setting the dropdown option in the shared axis section of the chart and selecting the name of the dimension instead of “Date Hierarchy”.

Doing this with our data results in a rather messy looking chart.

The data here is far too granular to display all of it across all of the available times. By default, using a date or datetime dimension on an X axis will display continuously like this. However, we can control this behaviour through an X axis property on the chart itself.

Opening up the chart display properties, and then opening the X axis section reveals that “Continuous” is selected for the Type property. This is the display mode that will scale the axis to include all available date/time values. The other option is “Categorical”. The Categorical option displays each date/time value as a discrete data element. Changing the axis type property to continuous results in the chart appearing as follows.

The continuous and categorical options are only available for date and date/time dimensions. If the dimension in the X axis is not one of these types, the “Type” option will not appear.

Using Continuous, each and every date and time value is displayed on the X axis, and the data values are clearly resolved. However, in our case, there are far too many values to make this useful. Finding what we’re after would take a lot of scrolling. It’s best in this case (and in most cases) to view the data in aggregate, which is to say totals and averages across different time periods, years, months, days etc. This is where the Date Hierarchy shows value.

Formatting with Date Hierarchy

Selecting our “DateAndTime” dimension back to “Date Hierarchy” immediately changes the chart to show all of the data aggregated by Year. It is also possible to see the detail of the hierarchy in the Shared axis property for the chart.

The top level of the hierarchy is shown, which is all of the data aggregated to the Year level.

I rarely use the “Quarter” level of the hierarchy, so I simply remove it, and have done so for the remainder of the operations. It can be removed simply by selecting the x beside it in the Shared axis property box.

If we want to see our data in a more granular fashion, we have three options – Drill down, Go down one level, and Expand all down one level, which are the icons listed left to right in the highlighter section in the image above. Drilling down is meant to be interactive. With Drill down selected, clicking on the data point in the chart will go down to the next level in the chart for that data point. It replaces the standard cross filtering or cross highlighting that would normally happen when selecting a data point. For example, with drilldown turned on, clicking on any column for 2019 results in the chart below.

Notice that the X axis now shows month names instead of the year. This cart is showing our measures by month now, but only for the year 2019. The up arrow in the upper left arrow can be selected at any time to go back up to year, or selecting one of the months will drill down further to show the values for all of the days in the selected month.

The second option, Go down one level behaves in a similar fashion, but it does not filter to the year, it simply takes the chart down one level in the hierarchy without first filtering by year. This could be useful when comparing months to each other in aggregate. The X axis changes in the same way as drill down, showing the values for that level of the hierarchy.

If we want to show the data more granularly than the year level, but we don’t want to aggregate all of the same month names together, we can use the third option – Expand all down one level, or as I like to call it, “drill down and out”. Selecting this option results in the chart below.

We can see the data broken out by both year and month in order. This is a much richer view and still understandable. For example, you can see that 2018 was generally warmer than 2019 due to the amount of cooling necessary at a glance. The title is automatically changed (if it wasn’t set manually) to reflect this configuration, and the X axis also shows both year and month.

In this particular example the X axis is still readable, but drilling down and out more than one level can be cumbersome, and very wordy. At the same time, you do need to know which year, month, and day a particular data point pertains to. The Z axis formatting pane has some further options that help with this. By default, all of the hierarchy levels are concatenated together when a hierarchy is expanded in this way. Going into the chart format tab, and selecting the X axis, we can see an option for this – “Concatenate Labels”. Turning this off presents each level categorically on different lines. This to my mind is much easier to read and is the configuration that I use.

The concatenate labels option only takes effect when a hierarchy is expanded past its root level.

The examples used above utilize a “Line and clustered column bar chart” but pertain to all of the standard visuals that employ an x and y axis.

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