Hideous prototypes are better at this stage anyway, keeps the focus on the concept and not the details.
My first thought on this one is to have three stacked line graphs, one for each type of defect with the stacks in each representing the severity groupings (low/medium/high) all sharing a single time axis.
You will have to include a legend somewhere for severity levels (would recommend a more red to blue scale instead of red to green to help color blind users).
You will also have to decide if you need to compare counts of bugs, violations, and code smells to each other. If so they need to share the same range on the y-axis. If not you can make the scales different and avoiding having one type with far more defects than the others from squashing the detail in the others.
The shortcoming of this design is that it is not optimized for comparing the same severities across defect types except for the very first severity (the red one in my mockup). This is because the yellow and blue stacked lines do not have a stable baseline and humans have a perceptual bias toward seeing the width of the band as the shortest line between the parallel edges instead of the actual vertical drop.
You can overcome it, but your at-a-glance interpretation is a bit off because of it.