在y轴上用小时设置ggplot上的ylim

Question

我正在绘制一些事件发生的一天中的小时。但是，我正在使用Shiny，我想事先设置图形的ylim。这是我到目前为止所尝试的内容

#my data
structure(list(Data = structure(c(15971, 15972, 15973, 15974, 
15975, 15978, 15971, 15972, 15973, 15974, 15975, 15978), class = "Date"), 
Nome = c("dummyName", "dummyName", "dummyName", "dummyName", 
"dummyName", "dummyName", "dummyName", "dummyName", "dummyName", 
"dummyName", "dummyName", "dummyName"), hora = structure(c(1382013987, 
1382014626, 1382015134, 1382014112, 1382013981, 1382013761, 
1382044587, 1382044676, 1382044398, 1382044961, 1382047646, 
1382044149), class = c("POSIXct", "POSIXt"), tzone = ""), 
tipo = c("entrada", "entrada", "entrada", "entrada", "entrada", 
"entrada", "saida", "saida", "saida", "saida", "saida", "saida"
), minHora = structure(c(1378090800, 1378090800, 1378090800, 
1378090800, 1378090800, 1378090800, 1378090800, 1378090800, 
1378090800, 1378090800, 1378090800, 1378090800), class = c("POSIXct", 
"POSIXt")), maxhora = structure(c(1378177140, 1378177140, 
1378177140, 1378177140, 1378177140, 1378177140, 1378177140, 
1378177140, 1378177140, 1378177140, 1378177140, 1378177140
), class = c("POSIXct", "POSIXt"))), .Names = c("Data", "Nome", 
"hora", "tipo", "minHora", "maxhora"), class = c("data.table", 
"data.frame"), row.names = c(NA, -12L), .internal.selfref = <pointer: 0x003524a0>)

# code to render my plot

p5 <- ggplot(myDf, aes(Data, hora)) + 
geom_line(aes(group=tipo))
p5

但是，我希望图形在某个值（例如00:00:00或06:00:00）开始ylim并以另一个（可能是固定的）值结束。我尝试使用scale_y_datetime，如下所示：

myDf$minHora <- as.POSIXct(paste(min(myDf$Data), "06:00:00"), format="%Y-%m-%d %H:%M:%S")
myDf$maxhora <- as.POSIXct(paste(min(myDf$Data), "23:59:00"), format="%Y-%m-%d %H:%M:%S")


p5 + scale_y_datetime(limits=c(min(minHora) , max(maxhora) ))    

这几乎是我想要的。唯一的问题是小时旁边的月份（10月）的名称。我真的不喜欢下个月的月份。

ps。：我在闪亮的时候使用ggplot。它可能没有任何区别。