The default on most charts. Ten dollars occupies the same vertical space whether price is at 20 or at 2,000.
Linear works fine for short windows and for instruments quoted in fixed increments, such as index futures over a single session, where percentage distortion is negligible. Traders who size positions in dollars per point often find linear more intuitive for placing stops.
The failure case is long history or large ranges. A trendline drawn across a decade on linear will be far too steep near the start and far too shallow near the end, producing false breakout signals. If price on your chart has more than roughly doubled, switch to log-scale before drawing anything.