If you want to zoom in or out on the price axis in thinkorswim, you can left-click and drag* on the y-axis to adjust it: dragging it upward expands zooms in to show greater detail but fewer prices, and dragging down zooms out on the chart. Once you make an adjustment this way, the scaling is placed in manual mode. Manual mode is when your cursor movement determines what's displayed as the high and low price of the chart. If you place the chart in automatic mode, the highest and lowest displayed price is used to automatically scale the y-axis to include both of those values.
It's easy to get into manual mode by just left-clicking and dragging on the axis, as explained above. But many users don't know the shortcut to quickly toggle back to automatic mode. The shortcut to go back to automatic mode is to double-click on the y-axis. This is faster than clicking on the autoscale icon near the top right of the chart, or going into the Chart Settings dialog to renable it via a popup dialog.
So in summary, click and drag on the y-axis to adjust the zoom level of the chart, and double-click on it to restore the y-axis scaling to automatic.
Note that these same features work on the x-axis as well.
* Holding down the CTRL key while dragging the clicked mouse up and down on the y-axis will pan the axis rather than zoom in and out.