When you create a line Chart in Calc it doesn't always display the lines.
Please attach a sample file, and paste the information in Menu/Help/About LibreOffice, there is an icon to copy.
Created attachment 189006 [details] spreadsheet that shows the line graph or chart bug this is a spreadsheet where the bug exists
The issue is with blank cells between data. With the chart line type, it's not allowed to select continuous line for Plot options in Data series options. Changing the line type for line with Points and Lines, you can see the points and edit the format for Data series. As the tip for that options mention, it is a default value for X/Y charts. Please take a look to LibreOffice help. file:///C:/Program%20Files/LibreOffice/help/en-US/text/schart/01/04060000.html?System=WIN&DbPAR=CHART&HID=modules/schart/ui/tp_SeriesToAxis/TP_OPTIONS#bm_id8064517 IMHO not a bug, I could a request for enhancement. *** This bug has been marked as a duplicate of bug 144746 ***