There is a gulp command to run a server (gulp server, which serves the root directory on http://localhost:8000) and an other to watch any changes made to the source files to then re-build a bundle js file (gulp default). I ran both these in separate terminals. These are not documented (I found them looking at the gulpfile.js).

The samples, such as http://localhost:8000/samples/charts/bar/vertical.html, include the uncompressed bundle that gets automatically changed every time a modification is made in any source file:

<script src="../../../dist/Chart.bundle.js"></script>

The bundle takes around 5secs to complete and after that reloading the sample page makes you see the changes.

This is not exactly what I was looking for, but for making small changes it should do just fine.

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