Bottoms-up approach in Hierarchical Clustering

To draw the horizontal line in the Bottoms-up approach:

1. Look for the longest vertical line that doesn't cross any extended horizontal lines. The length of this vertical line is called the Largest Distance.

In this step to check for the longest vertical line, imagine as if all the horizontal lines are extended and then identify the longest span.

2. Now take a threshold value that will cross this Largest Distance (the horizontal line is the threshold line) and calculate the optimal number of clusters.

This means that you draw the horizontal line (threshold line) on the Largest Distance. 

3. The number of clusters is equal to the number of vertical lines crossed by the threshold line.