Fix missing plots for 5 year time window

This only occurs for stations with only a few data points. All-time data works. 5 year window should have empty plots (no recent data), but instead fails to plot correctly.

Reproduce the failure with a station that has only a few data points and select the 5 year time window, then compare it with the working all-time view. Trace the plotting entry point used by that window and verify that stations with no recent data render empty plots rather than failing; confirm the all-time behavior remains unchanged.

Written by the indexing model from the issue text.