When creating a chart "Columns and Lines" the chart type page has a control "Number of lines". See attachment 199581 [details] of bug 165552 for an example of such chart. The control "Number of lines" has no help entry. The help entry is already missing in OOo 2.3. OOo1.1.5 has help entry: Number of lines Defines the number of lines for combination charts of bars and lines. The last n data series are displayed as lines, the others as bars. The SDK has: NumberOfLines Determines how many data rows are displayed as lines rather than bars. If this property differs from zero the last BarDiagram::NumberOfLines data rows are shown as lines. It is currently supported by two dimensional vertical bar charts only.
Olivier Hallot committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/commit/64bafdcd2618379cce19e997be54ee3b94c256fa tdf#167938 Help for number of lines in graph.