[R] date and time data on x axis

[snowball0916]

Hi, guys
How do you guys deal with the date and time data on x axis?
I have some trouble with it. Could you help with this?

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Sample Data
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The sample data look like this:

20181028_10:00:00 600 
20181028_10:00:01 500 
20181028_10:00:02 450 
20181028_10:00:03 660
......

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My Code
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library(lubridate)
mydata <- read.table("e:/R_study/graph_test2.txt")
xdata <- ymd_hms(mydata$V1)
ydata <- mydata$V2
plot(xdata, ydata, type="o")


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Questions:
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1. Why my x axis does not show me the correct date time like ""2018-10-28 10:00:00 UTC" ?
2. If my data is very huge(like data in every second and the data has the whole day , even the whole month), how can I display the x axis in a neat and clean way?

Thanks very much.
 



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