have you thought about explicitly noting the ability to filter using the 'affects dashboard filters'. One thing I'm doing more of these days is providing text boxes with guidance on how to use the dashboard, key things they should know. Recording a quick video could help too. I know that's not really about the widgets or visualizations themselves, but I'm increasingly focused on the guidance/training in the vehicle if I can.
If I think it is particularly important that people right-click to select, I note it in the title of the widget. Otherwise I include it in the widget description that shows up if you hover over the (i). And we have documentation for the dashboard linked on the dashboard.
So I've done some of that too, at least. I guess the part of 'but I don't know whether people notice' is also down to it being difficult to measure how much these things actually *get* used. Usage Analytics is a wonderful tool, but it doesn't say too much about filter useage.
