I have used px.line with great success, so I feel there may be something else in you usage of px.line that may be causing this. Could you paste the exact px.line command that you use?
Also, another important thing to consider is that this issue may be caused by the data in your data frame. I may not know exactly what I’m talking about, but my intuition tells me that plotly is just plotting the data in your column, and perhaps you need to clean up the data a little before it is ready to be plotted.