performance crashes with multiple calls of createisofill

[chaosphere2112]

I am repeatedly (17 times) calling a function which generates a set of four plots with VCS. They should all take about the same time. With normal use of VCS, they don't. Each is slower than the one before, and you get an overall slowdown of well over an order of magnitude - enough to make VCS almost unusable.

I narrowed the problem down to vcs.elements['isofill']. Each time my function is called another 10 or so names get added to this list. The running time of canvas.plot is superlinear in the length of vcs.elements['isofill'].

As a workaround, I saved a copy of an early form of vcs.elements, and restored it (other than vcs.elements['display']) frequently. This completely eliminated the performance problem, and my test problem's running time went from 3 1/2 hours to 16 minutes.

It seems to me that something in VCS does something very expensive with everything in vcs.elements['isofill']. There may be design issues - why does everything have to be saved forever? - but that is beyond my competence to judge.

Migrated from: CDAT/cdat#2135

[doutriaux1]

duplicate of #41