Ok, so 20 gravity entries work, but the 21st is not taken into account in the chart display, right?
There is no such a limitation in the plugin, and i tested with more than 20 entries this morning, but was not able to reproduce this bug.
What you see : id25pie for ex, is the id of a chart, so if the total is 20, it means you have 20 charts displayed on your page.