To set the range of a trend line in Excel, you can follow these steps:

1. Select the chart that contains the trend lines you want to modify.
2. Right-click on one of the trend lines and select "Format Trendline" from the menu that appears.
3. In the "Format Trendline" pane that opens on the right side of the screen, click on the "Options" tab.
4. Under "Forecast," you should see a section labeled "Forward" with a checkbox next to it. Uncheck this box.
5. Next, under "Backward," you should see a section labeled "Period." This is where you can set the range of the trend line.
6. Change the "Period" value to match the number of weeks that your data covers. For example, if you have data for weeks 1-6 and weeks 7-12, you would set the "Period" value to 6 for each trend line that covers weeks 1-6 and 12 for each trend line that covers weeks 7-12.
7. Click "Close" to apply the changes to the trend lines.

By setting the "Period" value in this way, you should be able to get the trend lines to extend just within the data range for each set of data.