Text in plot that has x-axis in terms of datetime

Hello,

I have a plot that uses a datetime structure as an x axis and I want to add a text string on my plot. Typically I would do that using text(x,y,'text'). In this case I am not sure what I would use as 'x'. The 'annotation' command uses normalized values that do not move when I zoom or pan the plot. Would appreciate some input on what I would use as my 'x' in the text function. Thanks!

Answer 1

[Ricardo]

I just had this problem and I could position the text using datenum in this way:

text(datenum( the_date ), y_axis_val, text)

[Dick Benson]

Excellent .... too simple !

Answer 2

[dpb]

A date number of the time x. Same idea as with ordinary axes; the date axes is one with the limits based on the first/last date of the x vector. You can retrieve these from the plot as

xlohi=xlim;

then scale between those two at whatever point you wish would be one way...

[George Mathai]

Thanks!

[dpb]

If (as I presume given the comment) this solved the problem, please "ACCEPT" the answer -- besides my ego, it notifies other users who find the question the answer was considered/is a good 'un, thereby (hopefully) helping fulfill the idea of the forum in being a database repository...

Answer 3

[Steven Lord]

Don't use datenum anymore. It is not recommended. Instead, use ruler2num or num2ruler to convert the datetime data into coordinates the text function can accept.

x = 1:10;
d = datetime(2025, 7, x);
y = x.^2;
h = plot(d, y);
value = ruler2num(d(5), gca().XAxis); % Convert July 5th to a number
text(value, 25, 'here')
grid on

The call gca().XAxis is a reference to the X axis of the current axes. ruler2num needs to know which ruler's coordinate system you're trying to convert into. If I wanted to be a little more careful (since which axes is the current axes can change easily, with a click in a different location in the figure or on a different figure) I could use the ancestor function to get the axes ancestor of the line created by plot. This is independent of which axes is current.

figure
h = plot(d, y);
ax = ancestor(h, 'axes');
value = ruler2num(d(7), ax.XAxis); % Remember d(7) is July 7th
text(value, 20, "y = 20 on July 7th")
grid on

You can see that the "y" starting the text lines up with July 7th (d(7)) on the X axis.