What is a graph scale?

The scales of a graph determine the reference points for data displayed on the graph. A graph scale includes a vertical or horizontal axis line, tick marks for specific values or categories, and tick labels.

Types of scales

Graphs can have several types of scales, often on the same graph on different axes. A continuous scale is a sequential numeric scale with an infinite number of points between values. A categorical scale displays distinct, related groups of data; the categories are equally spaced and the space between the categories has no meaning.

Continuous and categorical scales

This bar chart uses a continuous scale on the y, or vertical, axis to denote the degree of durability. The bar chart uses a categorical scale on the x, or horizontal, axis to identify the carpet types.

Other types of scales include time scales, which display equally spaced time units (for example, day, month, quarter, year), and probability or percent scales, which are logarithmic and show the probability or percentage of observations that fall at or below certain values.

X-, Y-, and Z-scales

When you plot two variables in Minitab, you usually display the y variable on the vertical or y-axis to represent the response and the x variable on the horizontal or x-axis to represent the predictor. When you plot variables in three dimensions, the x and y variables usually represent the predictor variables and the z variable usually represents the response.

Scatterplot with x and y scales

This scatterplot shows how reheat time (x) affects the quality (y) of a frozen entree.

3D scatterplot with x, y, and z scales

This 3D scatterplot shows how both temperature (x) and reheat time (y) affect the quality (z) of a frozen entree.

Modify the range of a continuous scale

Minitab fits the graph scales to the range of your data, but you can change the range.

  1. Double-click the axis that you want to change.
  2. On the Scale tab, under Scale Range, do the following:
    • Deselect Auto for Minimum and enter the new minimum value.
    • Deselect Auto for Maximum and enter the new maximum value.
    • If you want the x-and y-scales to be the same, select Same scale range for Y and X.
  3. Click OK.

Transpose the horizontal and vertical scales

For probability plots, bar charts, boxplots, interval plots, individual values plots, and histograms, you can transpose the horizontal and vertical scales.

Original
Transposed
Example of transposing the scales

In a default bar chart, the categorical scale is horizontal and the continuous scale is vertical. To emphasize the differences in the values, you can transpose the two scales.

  1. Double-click an axis and choose Transpose Y and X or Transpose value and category scales.
  2. Click OK.

Transform a continuous scale with a logarithm or power

On a scatterplot, time series plot, and area graph, you can transform a continuous scale with a logarithm or power to get a different view of your data.

Original
Transformed
Example of transforming a scale

In the original scatterplot, the simple fitted regression line does not accurately model the curvature in the data. When the horizontal scale is transformed to logarithmic values, the simple fitted regression line accurately models the data.

  1. Double-click the axis of the continuous scale and click the Transform tab.
  2. Choose Logarithm or Power, and specify the parameters.
  3. Click OK.

Add a secondary scale to an overlaid plot

If you have overlaid scatterplots or time series plots, you can add a secondary scale to display on the opposite side of the graph.

Original
Secondary Scale Added
Example of adding a secondary scale

On the original plot, the x variables (Height and Weight) share the same primary axes. The variables have different ranges, causing the data values for Height to be difficult to interpret. Assigning one variable to a secondary scale with its own range causes the data values for each variable to be easier to read.

  1. Double-click the axis and click the Secondary tab.
  2. Under Scale, choose the variable to use for the secondary scale.
  3. Click OK.

Change the time scale labels

On time series plots, area graphs, and control charts, the x-axis represents chronological time in equally spaced intervals. You can choose the labels to use for the equally spaced time units.

  1. Do one of the following:
    • To choose the time scale labels when you create a graph: in the dialog box for the graph you are creating, click Time/Scale or Scale and choose the Time tab.
    • To change the time scale labels for an existing graph: double-click the x-axis of the graph and choose the Time tab.
  2. Choose the Time Scale.
    Index(default)

    Use integers for labels on the x-axis scale. In this graph, 1 indicates the first observation, 2 indicates the second observation, and so on.

    Calendar or Clock

    Use calendar or clock units that you specify on the x-axis scale. In this graph, the units are months. (Not available for control charts.)

    Stamp column

    Use values in a stamp column for the x-axis scale. In this graph, the stamp column specifies the shift and day.

  3. Specify any other options, such as Units and Start Values.
  4. Click OK.

Change the y-scale type

You can change the y-scale type of histograms, probability plots, and empirical CDF plots. For example, by default, the y-scale of a histogram represents frequency (each bar represents the frequency of values within the specified bin), emphasizing the magnitude of each bin. If your audience doesn't have enough process knowledge to understand the frequency values, you can change the graph's y-scale type to recast these frequency values as percentages (each bar represents the percentage of all values within the bin), a format they may find more meaningful.

By default each bar represents the frequency of values within the bin. Change the y-scale type to Percent to make each bar represent the percentage of all values within the bin. Use Density when you want to compare distributions and the sample size differs. Density is also useful when you compare bars and the bin widths are unequal. Density is calculated as the proportion of observations divided by the bin width.

Frequency (default)
Percent
Density
  1. Do one of the following:
    • To choose the y-scale type when you create a graph: in the dialog box for the graph you are creating, click Scale and choose the Y-Scale Type tab.
    • To change the y-scale type for an existing graph: double-click the y-scale of the graph and choose the Type tab.
  2. Choose the Scale Type.
  3. Click OK.
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