support mathjax header in plotly backend

src/backends.jl

@@ -374,6 +374,7 @@ function _initialize_backend(pkg::PlotlyBackend)
     catch
         @info "For saving to png with the Plotly backend ORCA has to be installed."
     end
+    push!(_plot_defaults, :include_mathjax => "")
 end
 
 const _plotly_attr = merge_with_base_supported([
@@ -417,6 +418,7 @@ const _plotly_attr = merge_with_base_supported([
     :tick_direction,
     :camera,
     :contour_labels,
+    :include_mathjax,
   ])
 
 const _plotly_seriestype = [

src/backends/plotly.jl

@@ -813,6 +813,11 @@ function plotly_html_head(plt::Plot)
     local_file = ("file://" * plotly_local_file_path)
     plotly =
         use_local_dependencies[] ? local_file : "https://cdn.plot.ly/plotly-latest.min.js"
+
+    include_mathjax = get(plt.attr, :include_mathjax, "")
+    mathjax_file = include_mathjax != "cdn" ? ("file://" * include_mathjax) : "https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.7/MathJax.js?config=TeX-MML-AM_CHTML"
+    mathjax_head = include_mathjax == "" ? "" : "<script async src=\"$mathjax_file\"></script>\n\t\t"
+
     if isijulia() && !ijulia_initialized[]
         # using requirejs seems to be key to load a js depency in IJulia!
         # https://requirejs.org/docs/start.html
@@ -826,7 +831,7 @@ function plotly_html_head(plt::Plot)
         """)
         ijulia_initialized[] = true
     end
-    return "<script src=$(repr(plotly))></script>"
+    return "$mathjax_head<script src=$(repr(plotly))></script>"
 end
 
 function plotly_html_body(plt, style = nothing)