Did you intend to offer a colour picker for the histograms? I must not have looked at it before, but it doesn't offer anything.

I can see it on column, bar, and pie charts - either a one colour or multi colour picker.
Line charts have predetermined colours to go with their line displays.
I see the scheme on arc and network graphs.
So, it's just histograms I'm not sure about.

The NumericDataTypes module provides the "Histogram (time series)", "Line chart (time series)", and "Grouped line chart (time series)" diagram types. I plan to work on NumericDataType's data visualization implementation later in the year. Would you mind creating an issue in that module's repository to this effect?

Sure thing. This doc update is done, then!