Add a flamegraph

[SEJeff]

For many types of visualization, especially related to performance monitoring, a heatmap is one of the best visualizations. Consider adding this to d3.compose

[timhall]

These are definitely interesting! Should be very doable with d3.compose. We're looking into developing a collection of user-generated / custom charts and components that can be simply plugged in to your project and I'll make sure this is included with them.

(also, tangentially related to #34 in way, since a linear scale would fit better than ordinal with this type of chart)