Enable arbitrary number of columns per axis, configurable column "reduce type"

This allows adding an arbitrary number of columns to the display - foundation for stacked charts / multiline charts.

Columns are grouped by "axis" - each chart type can specify a number of axes which accept different kinds of columns. (Previously axis-column was 1-1 relationship)

It also adds alternatives to sum reducing.. namely count and list (list all encountered values for that x-axis value).