Thank you for posting.
At this time there is no feature built in to adjust the line thickness in the line chart type. As an alternative the area chart type is easier to see, but that is the only way to make the display easier to see visually at the current time.
I will forward your request to the appropriate department for consideration in a future release.
Also, I recommend that you go to the link below and fill out our product feature request form with both the feature you would like added, and an example of how it would be beneficial to a 'real world' user. This tends to give our developers a better idea of what people want out of the software and why it would help them.