Fixed Shiny app GUI issues related to downloading and axis labels.
Moved xlab and ylab arguments to hd_opts(), since they relate to figure presentation rather than data specification. Use NULL to hide axis labels. (#5)
Corrected ggplot2 color palette selection to follow the standard rules defined by resolve_colors().
Improved the overall visual appearance of the highdir Shiny app.
Implemented modules in the Shiny app.
Added a registry for optional arguments for specific geoms. List them using geom_args().
Shiny app now displays optional geom arguments along with explanatory comments.
Add arg percent to hd_opts() to show % in tooltip and y-axis if TRUE.
Add arg xtick to hd_opts to select column for custom x‑axis tick labels when these differ from the numeric x values. Note that Highcharts uses zero-based indexing, while R uses one-based indexing.