Word to describe adding an independent variable to a chart

Consider the following chart.

The word "series" is used to describe something unique about the data such as the city.

There is also something common to each series curve such as the month represented on the X axis. While this example used time as the basis of the X axis, another chart could have used something else such as elevation.

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Another chart might look like the following. For this one, the year is the series, and the country is the X axis (and country isn't even on the X axis for a bar chart but only a column chart so using that name isn't good!).

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For this one, the series is the person, and the x-axis is the fruit.

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For this one, the series is also the person, and the x-axis is the fruit.

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I am designing an application were the user can add a new series as well as a new X axis point. I am thinking of two buttons Add New Series and Add New X Axis.

I am thinking the user workflow will be as follows:

  1. User adds a new empty chart by selecting the type of chart they wish to add (i.e. line, pie, bar, etc) and giving it a name.
  2. User adds a series and gives it a name.
  3. User adds what I refer as the x-axis point and gives it a name. I think it will initially be added with an empty value (more on this later).
  4. User clicks the empty point, and can add a value.
  5. User repeats step 3 (and optionally step 4), and adds two more x-axis points.
  6. User adds another series. That series automatically has empty values for the three x-axis points.
  7. User deletes an x-axis, and the values are removed from the three series.
  8. User changes the chart type if they want.

Using Excel as a design pattern, I can create the following charts using the same data. For this, I called the independent variable "row". While this makes sense from the spreadsheet prospective, it doesn't make sense from most of the chart's perspective. What would a user think if there was an "Add New Row" or "Delete Row" button?

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I feel the word "series" works (correct me if you feel differently), however, I don't know a good word to describe the x axis. The reason I am using x-axis is that the JavaScript library Highchart uses it.

What word should be used in place of x-axis?

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