Do you want a trendline, or merely any old line connecting the points? A trendline gives a line of best fit, and in your case, the two middle points pulls this line away from the endpoints.
First of all, you probably want an XY Scatter chart, not a Line chart. The line chart plots each X axis item at an equal distance from its neighbors, so 10 is as close to zero as to 15. Here are the two different chart types:
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You don't need the markers, I just like using them so I can easily see where the actual data points are in my chart.
Second, whether you want a trendline, with its attendant statistical content, or simply a connecting line, the easiest way is to add a series that uses just the values for points K and Z, without the two intervening points. Then either use a series format that connects the points with a line, or hide the points (no marker and no lines) and add a trendline.
You need a data range like this (below left), with the X and Y values for the points you want to add. Select the range and copy it, then select the chart, and go to the Home tab of the ribbon, click the little down arrow on the Paste button, and choose Paste Special. Select the options shown below right.
A new series is added, as shown at bottom left. You can use it as is, or you can use whatever line color and other format you need. Or if you do want an actual trendline, hide the added data points (format with no markers or line), and add a trendline, shown here with the default dotted line.
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