Error Bars

Error bars are a way to express confidence in the accuracy of measured data. While a point on the graph expresses the actual value that was imperfectly measured, its error bars show how much higher or lower the true value could be.

To create error bars with the same positive and negative offsets for every point:

Select the points to add error bars to.
From the Arrange menu, choose Make Error Bars.
In the sheet that appears, enter the positive and negative offsets you want.

To create error bars with different offsets for each of a few different points:

Select one of the points.
From the Arrange menu, choose Make Error Bars.
In the sheet that appears, enter the positive and negative offsets you want for the selected point.
Repeat this process for each point.

If you have enough points that selecting them individually to assign error bars would be a royal pain, you can generate error bars from columns of data.

To create error bars from columns of data:

Make sure your data is set up as one column of x-values followed by three number columns: one for the data point, one for the bottom of the error bar, and one for the top of the error bar. (See the Importing Data section for more about data formatting.)
```
year	median	high	low
1940	52.4	59.8	43.2
1941	62.6	66.2	49.6
1942	54.9	66.5	51.6
1943	79.1	88.9	68.8
1944	97.6	104.6	90.6
```
Copy the data and paste it into your graph; you get three data sets.
Select all of the points and choose Make Error Bars from the Arrange menu. OmniGraphSketcher detects the highest and lowest points at each x-value, and turns them into error bars for the middle point.

Often, error is expressed as positive and negative offsets from a median or average, rather than as top and bottom values. If this is the case for your data, you need to convert them to the format OmniGraphSketcher expects. You could, for instance, use a spreadsheet formula to add or subtract the offsets from the median value for each row, and place the results in new columns.