How to convert datetime axis to show interval in days

I am trying to plot a line chart based on the data in the file attached but I want the x-axis to show intervals in days instead of the time date. How can this be done in Excel 2010? I tried to play around with the format axis but I can only see Horizontal (Categorical) Axis option and now Horizontal(Value) option to change it.

Thanks.

Your profile shows that you are using Excel 2007, but you say you are using Excel 2010. There's not a lot of difference between 2007 and 2010, but you might keep your profile up to date just in case it ever does make a difference.

The obvious problem that I see when I open your file is that you don't have any true dates in your spreadsheet. Sure, you have a column of text strings that look an awful lot like dates, but Excel is very stubborn about differentiating numbers/dates stored as text and true numbers/dates. In order to use a nice date axis in a line chart, you will need to have real date serial numbers in your spreadsheet and not text that looks like dates.

Strategies for converting numbers/dates stored as text:
Since dates are really just numbers with special number formatting, these strategies work: https://support.office.com/en-us/art...1-c5bad0f0a885
This page is for dates, and includes a discussion of the DATEVALUE() function: https://support.office.com/en-us/art...4-32a67ec0a680

If you are up to some extra reading, I would recommend something like this to learn how Excel uses a date/time serial number system for storing dates and times: http://www.cpearson.com/Excel/datetime.htm

I don't have Excel in front of me, and I don't remember the exact layout of the format axis dialog for a date axis. There should be a place to set your "base unit" to either day, month, or year. There should also be options for start date, end date, and how many days (months/years) between tick marks.

Finding those options in the dialog always seemed pretty obvious to me, so I wonder if I am misunderstanding something.

what i should do next to specify the intervals to according to days instead of the dates instead

I may have misunderstood until now. This suggests you want elapsed days along the horizontal axis rather than calendar dates. Charts don't analyze data for us. If you give a chart calendar dates for the horizontal axis, then it is going to show calendar dates. If you want elapsed days on the axis, then you will need to calculate elapsed days in the spreadsheet, and use that for the horizontal axis data. With your dates stored as serial numbers, this is a simple subtraction operation. =A2-$A$2 (note the mix of relative and absolute references) will give you elapsed days since the date in A2. If Excel decides to be "helpful" and format the cell as "date" it will give you dates in 1900, but the underlying value is correct. You will simply need to format the cells as general or fixed or something that is not a date. Then use that column as your horizontal axis data.

You are using a stacked* line chart with an automatic horizontal axis. Under these conditions (x axis data is formatted as regular numbers not dates), Excel is going to automatically choose a text axis. In order to be able to choose different axis divisions, your axis needs to be either a date axis (by forcing the date axis option in the format axis dialog), or you need to change your chart type to an XY scatter chart, where the horizontal axis is always a value axis. The decision will depend on exactly what kinds of divisions you want to be able to specify for your horizontal axis. If you are going to divide the axis up into a fixed number of days (7 days, 10 days, 30 days, or let Excel automatically choose), then changing to a scatter chart may be the easiest. If you want to be able to specify "calendar" type divisions (monthly tick marks), then you will want to leave it as a line chart with a date axis. My personal preference would be to use a scatter chart, but that's only my preference.

* Were you aware you were using a stacked line chart? It doesn't matter with only one data series, but I can't tell you how many times someone has come on here with this kind of line chart with multiple data series wondering why series 2-x are not plotting at the correct value. You might make sure you understand the difference between a regular line and a stacked line and make sure you are using the correct line chart for your application.