Your demo is ugly (Sorry, had to say it), but it works and I like the fact that you can pan around by clicking anywhere. That is not obvious however and might need a hint somewhere. The custom scrollbar is not intuitive because of the coloring, I kept thinking the track was the slider and vice-versa. After playing a bit more with it I discovered you have color themes. DEFINITELY use one of those by default. The grayscale theme is super boring and completely opposite of your goal which I believe is to "show off" your project schedule.

I think you should be able to pan by dragging the chart header (the months/years).

Turn on alternating columns in the demo. Also use the square or round corners but not the one in the middle.

Also the task tree could stand to be widened a bit by default, it looks weird that in your demo you start off with a bunch of tasks only showing halfway.

Expand/Collapse should be a single button, the icon of which changes to indicate state.

When you do a snapshot it is not clear what the output file is going to be I just got a file called "chart" - is that an Image? Is it a MS Project file?

I'm not sure you want to use the full blow "File/Edit/Format/View/Help" menu. It blends in too much with the chrome of the browser window and you would probably be better served (in terms of UI/UX) by using standard icons for the most commonly used items. Also add contrast to that menu/toolbar (Still talking only about file/edit/etc...) so that it doesn't blend with browser chrome so much.

The units on the "Zoom tool" in the bottom right corner are not obvious to me (the P and W)

Otherwise its looking really good. Keep at it!