GainCurvePlotC
plots a cumulative gain curve for the case where
items have an additional cost, in addition to an outcome value.
The x-axis represents the fraction of total cost experienced when items are sorted by score, and the
y-axis represents the cumulative summed true outcome represented by the items seen so far.
For comparison, GainCurvePlotC
also plots the "wizard curve": the gain curve when the
data is sorted according to its true outcome/cost (the optimal sort order).
To improve presentation quality, the plot is limited to approximately large_count
points (default: 1000).
For larger data sets, the data is appropriately randomly sampled down before plotting.