The underwater curve touches zero at each new equity high and dips below it in between. It is the same data as the equity-curve with the growth removed, and it is far more informative about what holding the strategy felt like.
Three readings matter. The deepest trough is max-drawdown. The widest gap between two zero-touches is the longest drawdown-duration, often more discouraging than the depth. And the proportion of time spent below zero is flat-time - many profitable systems are underwater 60-70% of the time, because new highs are by definition rare.
This is the chart to look at before committing to a strategy, and the one most marketing material omits. The ulcer-index is essentially a summary statistic of it.
Related: equity-curve, drawdown-duration, flat-time, ulcer-index