Numbers app: how to change x axis position + change chart's line width I have a regular linear graph and have y values running from negative to positive, enabling x axis shows the line at the bottom of the graph. How is it possible to set it to show at zero, i.e. running from the origin (0,0)?

I also wonder how to change the width of the line? I can change the size of the "data symbol", but can't find for the line itself.

UPD: This is easily possible to do in Excel, so it will be surprising if Numbers can't do it.
To answer the first part of your question, please see this Answer to Force line chart to show axis at point (0, 0)

Second part:
Select the line to highlight it.  Then under Style in the Inspector the Connection Line and weight can be adjusted by point.