> Some of my histories are placed in an IFrame object. I eventually realized that my code fails 
> when it tries to resize a history which is placed in an invisible IFrame. I resolved the issue 
> by making sure that I am resizing plots only if they are in a visible IFrame.

Just to be clear: You could resolve everything on your side, or do you need to change anything in mhistory.js?

Just a tip: IFrames are not good to put anything in. I recommend just to dynamically crate a <div> element, 
append it to the document body, make it floating and initially invisible. Then put all inside that div. Have
a look how control.js do it. This takes less resources than a complete IFrame and is much easier to handle.

Stefan