Select the y-axis. On the menu, select Format | Selected axis | Scale tab.
In the box labeled Category (X) axis crosses at:, enter the minimum value of
the y-axis.
Tip: If your y-axis goes from -10 to +10, you can enter -100000 in the
aforementioned box. Now if your data changes and your minimum value is -25,
your x-axis remains at the bottom.
Alternatively, you can keep the axis at zero, but put the labels at the
bottom. Double click the axis or labels to format the axis. On the
bottom-right of the patterns tab, choose Low for the labels.