How do I create a trend line in a line chart within ArcGIS Pro? For example, as well my line chart hitting each individual data point I would like an average trend.
These are your options within ArcGIS Pro
Types of tabular charts—Charts | ArcGIS Desktop
You can use matplotlib and python which comes packaged with pro, but it requires some scripting.
Examples can be found here
Matplotlib Examples — Matplotlib 2.0.2 documentation
hmmmm...OK, so you're stating I can NOT have a trend line out of the box?
the line chart or scatterplot... both require that the X-axis be a continuous variable. If you have discontinuities in time, you might have to produce separate xy pairs to plot those (I don't use arc* anything graphing so I haven't explored at all)
Other packages which you have access to with the ArcGIS Pro's anaconda installation (ie Matplotlib) have a much broader range of graphing options. You can also import other packages into the Anaconda distribution, but I wouldn't recommend going that route until you have to do more than the occasional graph
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