Add ability to define an intensity metric for heatmap

with a heat map, i can define my x and y axis, but i am limited to "count" as my intensity value. It would be good to be able to select any numeric column from the results.

No files or tests are named. Start by locating the heatmap implementation and the code that currently fixes intensity to count; then trace how x and y axis columns are selected. Done means a heatmap can use any numeric result column as its intensity value, with coverage for the selection behavior.

Written by the indexing model from the issue text.