Hi @jamesantony,
Could you please share a bit more about your scenario?
For your first question, do you want to show the Data label within the Line chart control? If you want to show the data label within the Line chart control, I afraid that there is no way to achieve your needs in PowerApps currently.
For your second question, do you want to change the line color within the Line chart control? If you want to change the line color within the Line chart control, please take a try with the following workaround:
- Select the Line chart control (rather than the group control)
- Modify the first RGBA() function within the color array in the ItemColorSet. e.g. modify the first RGBA() function of the color array as below:
RGBA(255, 0, 0, 1)

For your third question, do you want to change the size of the marker within the Line chart control? I afraid that there is no way to change the marker size of Line chart within PowerApps currently.
If you would like this these features to be added in PowerApps, please submit an idea to PowerApps Ideas Forum:
https://powerusers.microsoft.com/t5/PowerApps-Ideas/idb-p/PowerAppsIdeas
Best regards,
Kris