Areachart with the upper side filled is displayed when using secondary X axis and Y axis minimum property is greater than 0.
but when Y axis minimum property is 0 or less, lower side is filled.
Is it in the specifications?
Can you kindly create a JSFiddle reproducing the issue you are facing and share it with us so that we can look into the code, understand the scenario better and help you out?
We have just released v3.0 Beta-1 with this fix. Please refer to the release blog for more information. Do download the latest version from download page & let us know your feedback.