draw_svg: Y-axis height bug with title

When drawing an SVG of a tree sequence, and providing a title, the Y axis is not properly shifted downwards (note the position of 0 in the image below)

<img width="726" height="183" alt="Image" src="https://github.com/user-attachments/assets/e682868e-89f6-4d91-a10e-da1b465ef4ce" />

It is fine when drawing a single tree with a title.