missing image tests for %sqlplot

[edublancas]

we're not testing the outputs of the %sqlplot command

in #508, we'll add tests for %sqlplot bar and %sqlplot pie, but we need to add for the others.

the ggplot API has image tests, so we can use that code as reference

[edublancas]

I think we've added somet tests lately so we need to double check if we're missing tests

[mehtamohit013]

AC Criteria:

• Add matplotlib tests similar to %sqlplot bar and pie charts #508