Chart image quality in version 3.12.1 has decreased significantly.
Previously we were using 3.9.0 and the quality of the generated charts was good. After upgrade, we have noticed that charts quality went down significantly. Column bar edges, for example, is not sharp, but blurry. I have attached a screenshot with a Fiddle. The example is the one you have in your website.
After a investigation we have found the cause to be the x and y values of the <rect /> that are not integers. When the numbers are integers, the edges look fine. See attached image.
We have noticed that this happens on some resolutions only. From our experience, you will be able to reproduce the issue on 1920x1080 resolutions.