Plot changes when using datetime

My plot changes when I try to use a datetime vector for the x-axis. My y-value is a normal double vector. Why does my plot change when I use the datetime vector? First picture is plot(y). Second picture is plot(datetime,y).

Answer 1

[the cyclist]

It would normally be helpful for you to upload the data so we could investigate, but in this case I think it is pretty clear.

If you have, say, 100 data points in your y vector, then the command

plot(y)

will plot that y vector against

x = 1:100

and therefore each of your data points will be equally spaced along the x-axis.

But when you plot against datetime, then your datetime values must very differently spaced from each other. (For example, some may be a few minutes apart, but some are a day or two.) So, all of the y values in your two plots are the same, but their spacing along the x-axis is very different.

[Lisa Zell]

Thank you! That's the problem I guess. I am using stock market time data with a time gap between 4 p.m. and 9 a.m. the next day. Would you recommend to convert the dates to a text string and then match with the x-values?

[the cyclist]

It depends on how you want the data displayed.

If you want the data displayed with the overnight time-gap, you could insert a NaN value in the middle of the night, which will break up the line. Here is a simple example of what I mean:

rng default
x = [1 2 3 4 nan 6 7 8 9];
y = [rand(1,4) nan rand(1,4)];

figure
plot(x,y,'.-')

If you do not want the time-gap, and instead want 9am on Tuesday to immediately follow 4pm Monday, then I think I would do something like what you are suggesting, on the original plot.