Description of the problem

Please see the explanation to plot vertical and horizontal bars in the GMT documentation of the plot module, especially the following part (exemplary for horizontal bars):

Here, +i means we must accumulate the bar values from the increments dx, while +v means we get the complete values relative to base. Normally, the bands are plotted as sections of a final single bar. Use +s to instead split the bar into nx side-by-side, individual and thinner bars.

What is the expected bar plot (-Sb or -SB ) for only appending +v (no +s), maybe overlaid bars?
This question arises since the generated figure shown below remains unclear, especially for the lower bar. In the case of overlaid bars, three differently colored sections seem to make sense for the upper bar, since two of the four values for this bar are identical. The lower bar contains only three differently colored sections, despite four different values are given in the provided txt file. The values are not given in an increasing / a decreasing order.
Opening this issue is related to GenericMappingTools/pygmt#1521 (comment).

Full script that generated the error

Original post at GenericMappingTools/pygmt#1521 (comment).
Related PyGMT example at GenericMappingTools/pygmt#1521 (comment).

Data: data_vhbars.txt
Colormap (please remove the .txt): roma_vhbars.cpt.txt
CLI minimal example:

gmt plot data_vhbars.txt -R0/4/0/3 -JX8c/4c -SB0.75+v4 -Croma_vhbars.cpt -Ba1f1g1 -W1p,black -png vhbars_overlaid

Full error message

No error message occurs.

Actual outcome

System information

• Operating system: Windows-10-10.0.19043-SP0
• GMT version (gmt --version): 6.4.0