[Feature request] Use metric data as x, y axes.

[aaronleesmith]

It is often the case that we have dependent and independent metrics and want to observe the correlation between the two. There should be an option to set the x and y axis values to metrics and use the results as the axis values for the series.

For example, if I want to track the metric "Home cost" (the dependent variable) against the metric "Home size" (independent variable) I want home size to be the x-axis and home cost to be graphed.