Eric’s idea would probably be better. If someone selects all the categories on the left side panel, a day with lots of entries will completely cover the background.
Although, if someone has a long streak of high/low days, will all the borders make it look too busy?
I was thinking of colouring the bar that the actual date and the total occupy. However, that too may get lost in the shuffle if all categories are chosen, unless the total bar was set up to flash in that case. That might annoy folks though.
Perhaps lightly shading the total bar would work. It doesn’t really need to stand out like the bold colours of the categories, it just needs to indicate that there is something unusual about that particular day (or days).