In cumulative charts, before we create the scale, we need to actually convert our raw data into cumulative data.
This example is very barebones and for demonstration only. The actual process used by the cumulative
vis type is much more involved and handles actually setting values and remembering original states.
function preUpdateScale(self, data, mainData, compData) {
mainData = accumulateData(mainData);
compData = accumulateData(compData);
}