[FEATURE REQUEST] Option to choose the dot plot order [low to high expression as in Scanpy] about cellxgene

[slinnarsson]

As I explain in #2629 I think the proper way to plot single-cell data is to first plot the zeros in light grey, then plot non-zeros using a colormap, on top. As long as point overlap is kept to a minimum (often also an issue in cellxgene), this way of plotting accurately reveals both low and no expression.

For example, consider the difference for AQP4 expression in this example of glioblasts when using grey for zeros (left) or just using the colormap (right):

When just using a colormap gradient, the true structure of gene expression disappears. Plotting low-to-high will not make any difference to the plot.