Make the Zero line bold

Assume the axis is on the 0% point you could increase the weight of the axis

It would help if you posted workbook example of such.

If you can not use the axis then plotting data series is another alternative.

2 examples included.

First uses the horizontal axis

Second uses additional data series on secondary axis.
Secondary horizontal axis is unformatted so you can see it. Remove tick marks, ticklabels amd line.

That's because you original chart has no horizontal axis applied.

select the chart and use Chart Tools > Layout > Axes > Primary Horizontal Axis > Show Left to Right.

The format the axis to have thick coloured line style.
set Tick mark and tick labels to none.

Allanx,
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