Below charts are scatter plots of projects, with regression lines, without their outliers removed. (Removing outliers does not improve correlation.)
All projects show different phases as separate points. The “Phases grouped” chart shows multiple phases of a project as a single point with total Length, % of total tunnel length and weighted average of cost/km.
Hover over points for more info, zoom out to see the outliers. The charts have been cropped for easy comparison.