Strange Line Graph Output - Python

Question

I'm fairly new to Python and I've come up with an issue on a plot. Can someone explain why this line graph would connect this way? It seems like I am missing a key step in sorting the variables.

To clarify, if the data was set up in this fashion, how would one get a line graph with the points (1,2) moving diagonal to (2,5), down diagonal to (3,1) and then moving up to (4,2) and (5,4)?

import matplotlib.pyplot as plt

a = [3,1,5,4,2]

b = [1,2,4,2,5]

plt.plot(a, b)

Mathew - matplotlib is creating a line plot with co-ordinates (3,1) to (1,2) to (5,4) to (4,2) to (2,5) as it is taking a as the x-coordinate and b as the y-coordinate... it is taking (a[0], b[0]) as the first coordinate and so on.... What are you trying to plot? — Redox
– Redox, Commented Aug 17, 2022 at 1:37
To clarify, if the data was set up in this order how would one get a line graph with the points (1,2) moving diagonal to (2,5), down diagonal to (3,1) and then moving up to (4,2) and (5,4)? Sorry this should be my question and I'll edit appropriately. — Matthew Pardo
– Matthew Pardo, Commented Aug 17, 2022 at 1:42

Kirk Broadhurst · Accepted Answer · 2022-08-17 01:39:09Z

1

It appears as if line is simply joining your observations in the order presented; starting at (3,1) then to (1,2) and so on. This makes sense, maybe it's what someone would want, and should be easy enough to fix - just sort your data such that the X value is increasing.

It's easy to sort your a value; the catch is sorting your b value in the same way. To do this I'd zip the two together, sort, then unpack.

import matplotlib.pyplot as plt

b = [t[1] for t in sorted(zip(a, b))]
a = sorted(a)

plt.plot(a, b)

answered Aug 17, 2022 at 1:39

Kirk Broadhurst

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3 Comments

Hhadley Over a year ago

Why do you not have to add the key=lambda t : t[0]?

Kirk Broadhurst Over a year ago

@Hhadley you could do that, but it's not necessary. The default key for sort is just the object itself. Sorting a list of tuples is just like a sorting strings - first sort by the first element, then by the second, etc. In this case sorting by the first element is sufficient.

Hhadley Over a year ago

@KirkBroadhurst Oh ok that makes sense. Thank you! Never knew that about sorted. I'm sure that will be very useful in the future.

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