The Chart Object has to be on the layout that is current when your set field script step executes. Is it? (Or did you move the container to the chart when you should have moved the chart to the container?)
If you have the right layout up when this script executes and it still doesn't work, this would seem to be a bug and then should be reported over in Report an Issue.
The script is on the layout with the chart that is being copied. The container was on a different layout, but I just made another copy of the container and added it to the layout with the chart to see if the script was executing correctly without changing layouts.
This has to be the layout that is current when the set field script step executes or it won't work. That layout has to have the chart object on it. It does not have to have the container field on it but the layout's context has to work for that field reference to the container field.
But since it works in FileMaker Pro and not Web Direct, it would seem to be a possible bug...