Add key milestone dates to graphs

**Is your feature request related to a problem? Please describe.**
Feedback here to identify when a Minister's time in office started: https://x.com/bentlegen/status/1956315676689236330

**Describe the solution you'd like**
Graphs should have milestones for key dates denoted on them. As a start, let's use the date that that Minister came on to that file.

For the Prime Minister, that would be the date they were elected. For the Ministers, would need to check each one to see if they were on the file prior to the election.

In terms of UI, don't want the graph to become too crowded. Perhaps a more prominent dot or a mark on the x-axis. Check out how Mixpanel or Posthog treat [annotations](https://www.google.com/search?q=mixpanel+annotation&sca_esv=452a92eb2c2ade68&udm=2&biw=1280&bih=1334&sxsrf=AE3TifMBQnKgixyvbtb10vzpiGrNC5C_cA%3A1756926152095&ei=yJC4aJ_jBMmW5OMPprCamAw&ved=0ahUKEwif58P5o72PAxVJC3kGHSaYBsMQ4dUDCBE&uact=5&oq=mixpanel+annotation&gs_lp=EgNpbWciE21peHBhbmVsIGFubm90YXRpb24yBRAAGIAESMAMUKsCWPkKcAF4AJABAJgBeKAB8QeqAQM5LjO4AQPIAQD4AQGYAgygAoIIwgIHECMYJxjJAsICBBAAGB7CAgYQABgIGB6YAwCIBgGSBwM5LjOgB6ofsgcDOS4zuAeCCMIHAzcuNcgHDg&sclient=img#vhid=cFZP6HS88-257M&vssid=mosaic) for inspiration.