Chromatograms are displayed as dotted lines in Skyline when the Transition is "non-quantitative".
When the MS/MS Acquisition Method at "Settings > Transition Settings > Full Scan" is "DDA", then all MS2 transitions are considered to be non-quantitative, and all of the MS2 chromatograms will be drawn as dotted lines.
In your screenshot, it looks like a few of those chromatograms are solid lines, so it must mean that these transitions have been individually marked as non-quantitative.
There are a few ways that a transition could be marked as non-quantitative. If you right-click on a Transition in the Targets tree, there is a menu item to change whether they are quantitative. There is also a "Quantitative" column in the Document Grid that can be used to change whether the transition is quantitative.
Also, EncyclopeDIA spectral libraries sometimes tell Skyline that certain transitions are non-quantitative.

Non-quantitative transitions are used for peak picking and scoring, but their peak areas do not get added to things such as the "Total Area" value that is reported at the precursor level.

There is also a gap in the display of your chromatogram which probably comes from the fact that "Triggered chromatogram acquisition" has been checked at "Settings > Transition Settings > Instrument".
When "Triggered chromatogram acquisition" is checked, Skyline detects the time ranges over which the mass spectrometer was and was not acquiring data for the precursor, blanks out the parts of the chromatogram where data was not being acquired. Also, in these cases, Skyline does not do background subtraction. "Triggered chromatogram acquisition" is intended to be used with methods such as Thermo's SureQuant where the mass spectrometer has been told not to acquire particular MS2 spectrum until certain things are seen in the MS1.
-- Nick