Xaxis linewidth not behaving

When I try to increase the x-axis thickness, plotly overlays a new axis line (of the desired thickness), instead of just increasing the thickness of the existing axis line. Am I doing something wrong? For example if I edit the layout property ‘xaxis’:{‘linewidth’:3}, it produces a new line under the xaxis (above the xlabels) which has width3, but it looks bad. What can I do about this? I’ve tried by editing ‘zerolinewidth’:3 as well but this does nothing.