Improve history graphs for more accurate visualization

This update makes the history graphs display more accurate and continuous lines, even when data is sparse or has gaps. [#9]

• Graph lines now better reflect the actual data over time.
• Gaps are handled smoothly by carrying forward the last known value.
• Multiple values within the same time period are averaged for a clearer view.
• Added tests to ensure the graphs stay reliable in all scenarios.

This should make it easier to understand trends and changes at a glance.

Thanks to @angrycatmeowmeow for pointing this out.