When I look at the docs, heatmaps are treated as an extension of histograms. This means that all axes need to be either numeric or time.
Thing is, I have a data set where the best visualization is a heat map which includes a non-numeric Y axis (a series of labels to be sorted alphabetically). The x-axis is still time, and the z-axis is a numeric value. Is there any way to do this? Assigning a number to each label would be disruptive to the visualization, since then readers would be forced to consult a dictionary to know what the graph is about.