Thank you for your post.
At this time, it is not possible to change the line thickness in a line chart. For readability, I suggest changing to a different and brighter color scheme. For example, Jewel Tone.
I encourage you to enter the line thickness suggestion into our Feature Requests web form at:
These web entries are read by our Development and Product Management departments where they are discussed and considered for a future version. I could copy your post and paste it into the web form, but there are additional questions asked that only you can answer.