Disappearing segments in trend charts

[Myriam]

Hi, everyone,
I do not know why a trend chart is acting out.

I designed it with custom colors and thicker lines and when
I run my program, some of the line segments that form the
trend line disappear or become much lighter or thinner.

Does anyone know what's going on? If I reformat the line, just by
deselecting "custom" format and selecting it again, everything comes
back to normal. (i.e., the lines return to their normal thickness
and color).

Any suggestions?
Thanks!