u.series index of each MAIDR row (one row except for a line layer).
sourceIdxs[row][col] is the index into the series' data columns that
the MAIDR point at (row, col) was read from. For a scatter layer there
is one row, indexed by the layer's data index.
The scale the x values are drawn against.
The scale the y values are drawn against.
Where each MAIDR layer's marks came from, so a navigation position can be turned back into a uPlot series and data index.