This seems like a bug with Flex which I never noticed until just recently, but the vertical axis titles for CartesianCharts (and LineCharts as well) are blurry at higher resolutions. However this problem is not present with ColumnChart. I had to encode the charts with ImageSnapshot and export them as png files and I set the dpi to 300 and this is when I noticed this glitch. So maybe this also has to do with ImageSnapshot.
Have others noticed this problem and is there a way around it?