Add option for precipitation amount rather than probability

[mattrossman]

The graph seems to suggest that lots of blue equals lots of rain, but the chart just shows the chance of precipitation. Maybe it would be more useful to show the expected rainfall amount. Since there's no defined range of values for this, the bottom of the graph could be zero and the middle could be whatever an average amount of rainfall is. Naturally some values would go beyond the bounds of the layer. Or perhaps I could do some kind of log transform.