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.