Adding line to plot in qcc Control Chart

问题

I have a control chart below that I am plotting from the data random (data sample posted on the bottom),

All what I am trying to do is add a horizontal line that I specify the value of to this control chart. I tried looking at new.data to add in qcc but its plotting a complete new thing Please help me with this, thanks alot. The code below shows how I plotted it attach(random)

library(qcc)
E6LA <-qcc.groups(xdata,ydata)
obj1 <- qcc(E6LA, type="xbar",nsigmas=3, plot = "FALSE",
             confidence.level = 0.95)
summary(obj1) 
plot(obj1,restore.par = FALSE, type ="1", title = "Data for Stability per year",
      xlab = "Year_WW",ylab = "Stability")

The sample data is below posted:

   xdata       ydata
1  2016-01     63.87
2  2016-02     59.31
3  2016-03     78.60
4  2016-04     90.54
5  2016-05     83.04
6  2016-06     77.61
7  2016-07    131.60
8  2016-08     92.36
9  2016-09     77.75
10 2016-10     83.70
11 2016-11     93.76
12 2016-12     67.68
13 2016-13     77.04
14 2016-14     56.59
15 2016-15     68.91
16 2016-16     82.75
17 2016-17     64.29
18 2016-18    189.00
19 2016-19    162.75
20 2016-20    125.21
21 2016-21    140.89
22 2016-22     95.49
23 2016-23     76.52

来源：https://stackoverflow.com/questions/38056822/adding-line-to-plot-in-qcc-control-chart