VIS: Restrict Heatmap to 2 categorical columns

[dvdanielamoitzi]

Short description

Currently, it is possible to add multiple (more than 2) categorical columns to the heatmap via the settings panel, but the heatmap only shows 2 (one per axis), and ignores all others. So it would be best to restrict the categorical input to 2 in the heatmap, and make it clear somewhere in a tooltip that only 2 categorical attributes can be selected.

User stories

• As a user, I want to clearly see that I can only add 2 categorical attributes to the heatmap so that I'm not adding multiple categorical columns without having any effect on the visualization.