Add 'correlation heatmap' graphtype

Even though we are a timeseries platform, sometimes it is helpful to graph two different time series metrics against one another to visualize the relationship between them.
A correlation heatmap would graph two metrics against one another, and therefore would not have a time axis. I believe our heatmap visualization is currently capable of performing this visualization except that a lot of our visualizations assume that the graph has a time axis.

Start by locating the existing heatmap visualization and the code paths that assume every graph has a time axis. Determine how two metrics should be supplied and rendered without that axis; done means a correlation heatmap can visualize their relationship while existing time-series visualizations continue to work.

Written by the indexing model from the issue text.