PS: Is the general case of a background image handled by Uniplot, and does Uniplot allow for the idea of taking a PDF, putting the plot data into it, and then creating a new PDF file ?
Thanks for the question! So the labels only depend on the limits of the plotting window, which you can supply as a keyword option. In that sense, and together with using the same gridlines options, you could have the same background in some sense.
What uniplot cannot do today is to have multiple vertical axis (like weight and head circumference in your example).
Hope this helps!
P.S.: Regarding your PDF question, for that in fact I think that some plotting library to graphics (rather than the terminal) might be better
So, the question isn't really about the axes. The question is about the curved lines that are part of the background. These lines allow a medical person to know what the "normal" curves are for a child, and then the plotted points are compared to how well they match the curves (by observation) This tells the medical person if the child is growing normally, or if they are malnourished or have some other problem that should be watched to explain the unusual lack or presence of growth.
Since the data points may be gathered at any time, the possible creation of a combined PDF will allow the medical record to include which chart was used and when, so that folks in the future have a better handle on the history of that child.