I'm not sure if I understand exactly what you're trying to do, but I think you can achieve what you need by enabling vertical gridlines in your chart. You can then format the vertical gridlines to be bolder by changing their color and thickness settings.
To switch on the vertical gridlines, select the chart, click the Chart Elements icon and then click Gridlines > Primary Major Vertical.
You can see a walkthrough of this in the video lesson: Essential Skills Lesson 5-21 Add gridlines and scale axes.
To format the gridlines, right-click on one and click Format Gridlines... from the shortcut menu.
You can see a lot more about formatting chart elements in the video lesson: Essential Skills Lesson 5-7 Manually format a chart element.
I have also attached an example workbook containing a chart with vertical gridlines:
I hope this is what you were looking for, but please feel free to reply if you need any more help with this.
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