LibreOffice 5 Calc Linear Regression Graph change variable f(x) and x to something else
 

I've been racking my brain for hours trying to figure out how to do that. Does anyone know how or is it possible in LibreOffice? I'd hate to have to go to Microsoft Office for this.

I need to change f(x) and x to particular symbols like sigma.

Take a look at the Calc help files on creating a chart legend, and how to assign the legend symbols to particular lines on the chart.