I’ve plotted a polygon on a set of ternary axes using python-ternary and joined up the vertices to form a region on the plot, however I’d like to be able to fill this region so that it is a shape rather than just the outline.

I used the following code to generate the axes and a set of vertices for the polygon:

import ternary
import numpy as np

# Define the scale (the sum of the three components for each point)
scale = 100

# Define the vertices of the polygon (each point should sum to the scale value)
polygon_vertices = [
    (20, 30, 50),
    (40, 30, 30),
    (30, 60, 10),
    (10, 50, 40)
]

# Create a figure with a ternary axis
figure, tax = ternary.figure(scale=scale)
tax.set_title("Ternary Plot with Polygon Contour", fontsize=20)

# Plot the vertices of the polygon on the ternary plot
tax.scatter(polygon_vertices, marker='o', color='red', label='Vertices')

# Connect the vertices to form the contour of the polygon
for i in range(len(polygon_vertices)):
    next_i = (i + 1) % len(polygon_vertices)
    tax.plot([polygon_vertices[i], polygon_vertices[next_i]], color='blue', linewidth=2, linestyle='--')

# Fill the polygon area using the ax.fill method
tax.ax.fill(*zip(*polygon_vertices), color='lightblue', alpha=0.8, label='Polygon Area')

# Set axis labels
tax.left_axis_label("Component 1", fontsize=15)
tax.right_axis_label("Component 2", fontsize=15)
tax.bottom_axis_label("Component 3", fontsize=15)
tax.get_axes().axis('off')

# Set the gridlines
tax.gridlines(color="blue", multiple=10)

# Set ticks and gridlines
tax.ticks(axis='lbr', linewidth=1, multiple=10)
tax.gridlines(multiple=10, color="black")

# Display the legend
tax.legend()

tax.get_axes().set_aspect(1) #sets as equilateral triangle, needs to be the last step of plotting
tax._redraw_labels() #see above

# Display the plot
tax.show()

I get the resulting plot shown here:

As seen in the plot, it does plot the filled polygon but not in the correct place, and it plots two of them. I’m not sure why. How do I get the correct region filled?

Note: This is not the same question as Python fill polygon as this is Cartesian coordinates. When I try and plot with add_patches, I get a Type Error as Polygon can only take two positional arguments but I provide it with three, despite the earlier conversion of ternary to cartesian coordinates.