No worries!
Yes, effectively the issue is that the y-axis for the grammar portion of the chart is the sum of grammar + vocab reviews. This makes the chart difficult to read without hovering as the grammar potion isn’t at the correct y-axis.
Without hovering, taking a look at my chart for Sunday 13th - you’d get the impression I must have done just under 50 grammar reviews:
However, once hovering you can see the grammar dot is actually too high - I actually completed only 12 grammar reviews that day:
What I’d be expecting is that the y-axis for the grammar reviews is at 12 for Sunday 13th, which is why I’d consider it to be a bug/defect as the wrong value is being assigned.