Any idea what's causing this or how to fix it?
In a line chart showing multiple series plotted over time, many of the
points are plotted at the wrong level (wrong height along on the Y axis),
although the data value is correct (as shown in the "tool tip" when hovering
over the plotted point).

For example, here are consecutive values for 3 series:
9, 2, 3, 3, 2 plotted as: 14, 7, 4, 5, 4
4, 4, 1, 1, 1 plotted as: 5, 5, 1, 2, 2
0, 1, 0, 1, 1 plotted as: 1, 1, 0, 1, 1