histogram returns an error for vector data

When I try to call the histogram function I get an error.

Here's an example using a simple vector

xtest = [1 2 3 4 5]

histogram(xtest)

Here is the error message:

Error using histogram (line 172)

Values plotted against x-axis must be datetime values. To create datetime values, use the

DATETIME function.

Answer 1

[Steven Lord]

You can't create a histogram from double data in an axes in which you've already plotted datetime X data.

v = 0:5;
t = datetime('today')+days(v);
plot(t, v.^2)
hold on
histogram(v) % will error

Create your histogram in another axes, one that you haven't plotted into yet or one in which you've plotted using double X data.

[Klark Ahmeti]

What does your answer even mean?

[Steven Lord]

Let's say you have an axes in which you've plotted date and time-based data.

v = 0:5;
t = datetime('today')+days(v);
plot(t, v.^2)
hold on

If I were to call histogram to add that plot to this axes, it would select bins centered around 0, 1, 2, 3, 4, and 5.

[counts, edges] = histcounts(v)

counts = 1×6

1 1 1 1 1 1

edges = 1×7

-0.5000 0.5000 1.5000 2.5000 3.5000 4.5000 5.5000

Where exactly should the bin edges [-0.5, 0.5] be plotted on this date and time axes? You could argue that the value 0 should map to midnight today (00:00 on July 13th, 2022). But should 0.5 be marked as half a day after that (12:00 on July 13th, 2022), half an hour after (00:30 on July 13th, 2022), half a year after (00:00 on January 13, 2023), or half of some other time unit?

Rather than try to guess, MATLAB will error and require you to create your histogram from datetime data.

figure
v = 0:5;
t = datetime('today')+days(v);
plot(t, v.^2)
hold on
[counts, edges] = histcounts(t, 6)

counts = 1×6

1 1 1 1 1 1

edges = 1×7 datetime array
   12-Jul-2022 21:00:00   13-Jul-2022 18:00:00   14-Jul-2022 15:00:00   15-Jul-2022 12:00:00   16-Jul-2022 09:00:00   17-Jul-2022 06:00:00   18-Jul-2022 03:00:00

Now the bin edges are datetime values, and that MATLAB can easily plot since the X axis is already in units of dates and times.

histogram(t, 6) % Using the datetime data not the double data with 6 bins