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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Nyquist plot with imaginary poles, semi-circles only working after 'trick'","articleBody":"I'm plotting the Nyquist diagram of a transfer function which has poles on the imaginary axis:\r\n\r\n```\r\n\r\nimport control\r\nimport matplotlib.pyplot as plt\r\nimport numpy as np\r\n\r\nK = 1\r\n\r\nGp =  control.tf([1], [1,1,0,0,0]) \r\nGc = control.tf([K], [1]) \r\n\r\n\r\ncontrol.nyquist_plot(Gp*Gc,plot=True,omega_num=1000)\r\nplt.show()\r\n\r\n```\r\nWhich results in the following incorrect plot, that does not show infinte semicircles, but something incorrect:\r\n![image](https://user-images.githubusercontent.com/11015732/115715659-a683f180-a378-11eb-9f3f-99efe15967f4.png)\r\n\r\nAlso using `indent_direction` and `indent_radius` did not solve this.\r\nThe trick I found that makes it work is if I add `omega_limits=[0.001,10000]` to nyquist_plot:\r\n\r\n![image](https://user-images.githubusercontent.com/11015732/115715678-ab48a580-a378-11eb-96fc-5eac7222df88.png)\r\n\r\nCan you please fix such that the incorrect 'line' does not appear? ","author":{"url":"https://github.com/kevindekemele","@type":"Person","name":"kevindekemele"},"datePublished":"2021-04-22T12:42:16.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/613/python-control/issues/613"}