[pgAdmin][RM5716] AJAX to generate the charts continues to fire after we disconnect the server

Hi Hackers,

Attached is the patch to fix the issue mentioned in the subject.

Regarding hiding the color legend when the server is disconnected, I feel it's good to have the legend visible as it says it's a graph. Otherwise, a box with error is ambiguous. Plus, it's not going to change anyway even if graph data is available or not.

Please review.