Lines end on different points.

Hello guys, I'm new to this forum, therefore I would like to apologise if this post happens to be in the wrong department so to say.

I'm experiencing trouble with a Line chart in Excel 2003.
For a school assignment we have 10 different thermometers, who monitor the temperatures of different areas at different times.
These are all saved in an Excel file automatically.

Now I have been making line charts of this data, but not all thermometers generate the same amount of measurements.
So thermometer 1, for example, might have made 300 measurements and thermometer 2 might have made 400.

Due to this, the line of the different Thermometers end up ending in different positions (most measurements ends last, least measurements ends first).

I have been trying to get it connected to time, by using the option to format cells to time and by making the horizontal axis time bound as well,
but the very same problem same to arise every time.

Any ideas?

Thanks

Sorry for the spelling mistakes, not a native speaker