如何更改 R 中日期时间序列的 x 轴标签间隔

Question

我创建了一个绘图，其中 x 轴为时间序列，y 轴为计数值。我想知道如何更改时间序列 x 轴上的间隔，以便时间间隔为每 3 小时一次。我不确定如何将像 at=seq(0,100,10) 这样的简单方法应用于时间序列。其次，是否可以在数据集中未表示的时间使用文本标签？即下面的示例数据从 09:29 开始以 10 分钟为间隔，但是否可以使用 x 轴上的时间值落在小时上？我假设这样做的唯一方法是手动列出文本字符串中的时间，但想知道我是否遗漏了什么？任何建议将不胜感激。

示例数据：

> dput(df)
structure(list(datetime = structure(c(1396603740, 1396604340, 
1396604940, 1396605540, 1396606140, 1396606740, 1396607340, 1396607940, 
1396608540, 1396609140, 1396609740, 1396610340, 1396610940, 1396611540, 
1396612140, 1396612740, 1396613340, 1396613940, 1396614540, 1396615140, 
1396615740, 1396616340, 1396616940, 1396617540, 1396618140, 1396618740, 
1396619340, 1396619940, 1396620540, 1396621140, 1396621740, 1396622340, 
1396622940, 1396623540, 1396624140, 1396624740, 1396625340, 1396625940, 
1396626540, 1396627140, 1396627740, 1396628340, 1396628940, 1396629540, 
1396630140, 1396630740, 1396631340, 1396631940, 1396632540, 1396633140, 
1396633740, 1396634340, 1396634940, 1396635540, 1396636140, 1396636740, 
1396637340, 1396637940, 1396638540, 1396639140, 1396639740, 1396640340, 
1396640940, 1396641540, 1396642140, 1396642740, 1396643340, 1396643940, 
1396644540, 1396645140, 1396645740, 1396646340, 1396646940, 1396647540
), class = c("POSIXct", "POSIXt"), tzone = "UTC"), count = c(46, 
45, 47, 43, 49, 46, 46, 44, 44, 44, 45, 49, 50, 46, 45, 43, 47, 
48, 48, 46, 47, 46, 46, 44, 46, 44, 45, 49, 44, 46, 44, 48, 44, 
42, 44, 50, 44, 44, 48, 43, 43, 46, 44, 49, 49, 44, 42, 45, 45, 
48, 46, 45, 46, 43, 40, 50, 39, 42, 48, 44, 44, 46, 43, 47, 46, 
41, 43, 43, 46, 48, 45, 48, 43, 42)), .Names = c("datetime", 
"count"), row.names = c(NA, -74L), class = "data.frame")

>head(df)
             datetime count
1 2014-04-04 09:29:00    46
2 2014-04-04 09:39:00    45
3 2014-04-04 09:49:00    47
4 2014-04-04 09:59:00    43
5 2014-04-04 10:09:00    49
6 2014-04-04 10:19:00    46

示例代码（绘制所有日期时间）：

#Convert datetime character string to POSIXct format
df$datetime = as.POSIXct(strptime(df$datetime, format="%d/%m/%Y %H:%M:%S"), tz="UTC")

plot(df$datetime, df$count, axes=FALSE)
axis(2,las=2)
axis.POSIXct(1, at=df$datetime, labels=format(df$datetime, “%H:%M")) #datetime format can be changed as desired
lines(df$datetime, df$count, col="grey22")

Answer 1

首先，让我们更正您的axis.POSIXct 电话：

axis.POSIXct(1, at=df$datetime, format="%H:%M")

参数format 让您定义您的时间格式，而无需一一设置每个标签的格式。 at 参数是我们将用来定义刻度线的位置。

首先，您可以使用应用于 POSIXt 对象的seq 定义日期序列（请参阅?seq.POSIXt），其中可以为参数by 提供自定义名称，例如"3 hours"：

一个字符串，包含“sec”、“min”、“hour”、“day”、“DSTday”、“week”、“month”或“year”之一。这可以选择在前面加上一个（正或负）整数和一个空格，或者后面跟着“s”。

因此：

seq(df$datetime[1], tail(seq$datetime,1), by="3 hours")
# [1] "2014-04-04 09:29:00 UTC" "2014-04-04 12:29:00 UTC" "2014-04-04 15:29:00 UTC" "2014-04-04 18:29:00 UTC" "2014-04-04 21:29:00 UTC"

现在要使序列从一小时开始，需要使用round（请参阅?round.POSIXt）：

round(df$datetime[1], "hour")
# [1] "2014-04-04 09:00:00 UTC"
seq(round(df$datetime[1],"hour"), tail(df$datetime,1), by="3 hours")
# [1] "2014-04-04 09:00:00 UTC" "2014-04-04 12:00:00 UTC" "2014-04-04 15:00:00 UTC" "2014-04-04 18:00:00 UTC" "2014-04-04 21:00:00 UTC"

总而言之，您的代码变为：

df <- structure(list(datetime = structure(c(1396603740, 1396604340, 1396604940, 1396605540, 1396606140, 1396606740, 1396607340, 1396607940, 1396608540, 1396609140, 1396609740, 1396610340, 1396610940, 1396611540, 1396612140, 1396612740, 1396613340, 1396613940, 1396614540, 1396615140, 1396615740, 1396616340, 1396616940, 1396617540, 1396618140, 1396618740, 1396619340, 1396619940, 1396620540, 1396621140, 1396621740, 1396622340, 1396622940, 1396623540, 1396624140, 1396624740, 1396625340, 1396625940, 1396626540, 1396627140, 1396627740, 1396628340, 1396628940, 1396629540, 1396630140, 1396630740, 1396631340, 1396631940, 1396632540, 1396633140, 1396633740, 1396634340, 1396634940, 1396635540, 1396636140, 1396636740, 1396637340, 1396637940, 1396638540, 1396639140, 1396639740, 1396640340, 1396640940, 1396641540, 1396642140, 1396642740, 1396643340, 1396643940, 1396644540, 1396645140, 1396645740, 1396646340, 1396646940, 1396647540), class = c("POSIXct", "POSIXt"), tzone = "UTC"), count = c(46, 45, 47, 43, 49, 46, 46, 44, 44, 44, 45, 49, 50, 46, 45, 43, 47, 48, 48, 46, 47, 46, 46, 44, 46, 44, 45, 49, 44, 46, 44, 48, 44, 42, 44, 50, 44, 44, 48, 43, 43, 46, 44, 49, 49, 44, 42, 45, 45, 48, 46, 45, 46, 43, 40, 50, 39, 42, 48, 44, 44, 46, 43, 47, 46, 41, 43, 43, 46, 48, 45, 48, 43, 42)), .Names = c("datetime", "count"), row.names = c(NA, -74L), class = "data.frame")
plot(df$datetime, df$count, axes=FALSE)
axis(2,las=2)
axis.POSIXct(1, at=seq(from=round(df$datetime[1],"hour"),
                       to=tail(df$datetime,1),
                       by="3 hours"),
                format="%H:%M")
lines(df$datetime, df$count, col="grey22")