black lines covering graph

Hi there, I have a problem where a graph produced by my code has black lines covering it (see image attached). There doesn't seem to be any errors with the data, in fact you can make out the data on the graph behind the lines, and I have used this code three times before to produce these graphs and there has been no issues.
Does anyone know why R studio would produce these lines on top of data?
Many thanks.

Hi, welcome!

We don't really have enough info to help you out. Could you ask this with a minimal REPRoducible EXample (reprex)? A reprex makes it much easier for others to understand your issue and figure out how to help.

If you've never heard of a reprex before, you might want to start by reading this FAQ:

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