I created a really simple chart based on 2 columns (date and numeric value).
I click on the chart, go to Chart Tools/Layout. The Trendline drop down is greyed out.
What am I missing?
Excel – Why can’t I add a trendline to a chart in Excel 2007
chartsmicrosoft-excel-2007
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Similar to what petersohn said, can you use an XY chart with two data sets - the first being the your current data that you're currently representing with the stacked bar chart, and the second being your horizontal line?
You could recreate your current graph by using error bars in an XY chart instead of the stacked bar chart. Just turn off the line that connect the data points.
The horizontal line is easy - make a second data set with two points, the y value being your calculated value for the horizontal line, and the x value being the max and mins of your graph.
This may call for a stacked clustered column chart, where the value for "interrupted pregnancy" can be shown as a separate bar besides the stacked bars. The technique and its application without code is explained in detail on Jon Peltier's page.
http://peltiertech.com/WordPress/clustered-stacked-column-bar-charts/
The technique involves laying out your data in a specific way and "fudging" (technical term!) the chart settings to display the final result.
He also offers a paid tool to automate the creation of such charts.
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