A warning is harmless, but let the user knows what he might run into if something(multiple tools in this case) is not correct. A user who finds something wrong/not acceptable in his plot(with the tools) can fairly understand the warning, and act onto it. The one who finds multiple tools acceptable and correct in his plot will simply ignore this warning, as his work is getting done anyways. I guess issuing a warning shall do(atleast for present case). We can come up with a better solution to handle this situation(I am happy to work on it), but merging this PR will not have any negative effect on future work in this enhancement. It can be treated as a patch work until something more acceptable comes up. Please excuse me if you people think I am pushing it too hard (it's just my first attempt at development at bokeh, and I am not fully aware of its ideology)!