Is there any way to smooth the line in a combined column and line chart?
I made a test using LibO 126.96.36.199. According to my results it is not possible to have smoothed line curve in combination with a column chart.
May I propose you file an enhancement request here: https://www.libreoffice.org/get-help/bug/?enhancement=yes
You can insert a trend line, then, if desired, make the original line invisible. The trend line can be linear, logarithmic, exponential, power, polynomial or moving average. Choose the one that works best for you.
To insert a trend line, double-click the chart and click on the line so it is selected then right-click to bring up a dialog. Choose Insert Trend line, then make your selections. To make the original line invisible or change how it is displayed, select the line, right-click and choose Format Data Series in the pop-up dialog. Set the line style, color, width, etc.
When I made the line invisible I had a hard time finding it to select it again (so be cautious).