Serves the vendored plotly.js bundle (the offline fallback) from
priv/plotly_js/plotly.min.js.
Mount it in your router so the fallback URL resolves:
forward "/plotly_ex", Plotly.Phoenix.PlugThis makes /plotly_ex/plotly.min.js available. The last entry in
Plotly.PlotlyJS.source_urls/0 adds a vendored-version query parameter so
the immutable browser cache requests a fresh bundle after upgrades.