[R-sig-teaching] Graph Two Series over Time

Thu Dec 24 16:49:59 CET 2015

Steve,

This is on the edge of what R-sig-teaching is for (since it isn’t really about teaching).  But since I think there are elements of what you are doing that lead students to think that R is terrible, I’ll show you how I might approach things.

First a few comments about my solution.

1) I generally avoid loose vectors.  I prefer to use data frames to keep related vectors related.

2) I prefer to code dates as dates.  I would be very nervous about code that manually sets the axis labels differently from the data.  That can lead to all sorts of bad errors down the road if you change the data and forget to change the labels and often indicates you don’t have the data formatted the way you should.  (Note:  I added day of month values to your dates that had none.)  The lubridate package makes it easy to create dates from strings.

3) I rarely use base graphics, so I’ll show you solutions using lattice and ggplot2.  There may be nice ways to do this in base graphics as well.

4) I’m ignoring the color choices, title, etc.  All that can be easily added, but I’m focusing on getting the data display correct.  That’s generally the approach I take to plotting:  First get the data display correct, then fancy up titles, colors, fonts, etc.  It’s saves lots of times, because often once I see the plot, I realize it isn’t what I need, so there is no reason to gussy it up.

5) I prefer (and lattice and ggplot2) encourage keeping the data manipulation in one location and the plotting after that rather than going back and forth between those two types of operations.  I find that it makes the code easier to read.

6) One of your series as fewer points than the other.  I made the assumption that the missing value was at the end.  That should be changed to whatever is correct for your data.

7) I don’t know what you were using pch to indicate, so I created a variable called “group” with values 0 and 15.  The variable and its values should ideally be renamed to reflect what they represent.  That will make your code easier to read and produce better labeling of the plot.

And one note about your code.

6*0:max_y

probably doesn’t do what you expect since the 6 does nothing here (because 6 * 0 = 0).  You could do 6 * (0:max_y), but isn’t clear why you would want the range of the plot to be six times that of the data.  Maybe you were thinking something like seq(0, max_y, length.out = 6), but that will give pretty ugly breakpoints.  In any case, the plots below do a fine job of setting the axes by default, and each system allows you to tune them if you disagree with the default for a particular plot.

With that much preamble, the code is now shorter than the introduction.

### Put data into a data frame -- avoid loose vectors
library(dplyr); library(lubridate)

# if i knew what you were using pch for, i would name group and its values to match
MyData <- data_frame(
  Well1 = c(0.005,0.005,0.004,0.006,0.004,0.009,0.017,0.045,0.05,0.07,0.12,0.10,0.20,0.25),
  Well2 = c(0.10,0.12,0.125,0.107,0.099,0.11,0.13,0.109,0.10,0.115,0.14,0.17,0.11,NA),
  dateString = c("1Jan05","1April05","1Jul05","1Oct05","1Jan06","1March06","1Jun06","2Oct06","17Oct06","1Jan07","1April07","1Jul07","1Oct07","1Dec07"),
  date = dmy(dateString),
  group = factor(c(0,0,15,15,0,15,15,15,15,15,15,15,15,15))
)

## using lattice
## lattice makes plotting two series easy
## but doesn't make it as easy to have different symbols along the same series

library(lattice)
xyplot(Well1 + Well2 ~ date, data = MyData, type = c("p","l"), auto.key = TRUE)
## better legend
xyplot(Well1 + Well2 ~ date, data = MyData, type = c("p","l"),
       auto.key = list(points = TRUE, lines = TRUE))

## using ggplot2
## for highly customized plots, i generally find ggplot2 works better
## i would reshape the data with tidyr before plotting (could be don in lattice as well)

library(ggplot2); library(tidyr)

MyData2 <-
  MyData %>%
  gather(location, concentration, Well1, Well2)

ggplot( data = MyData2, aes(x = date, y = concentration, colour = location)) +
  geom_line() +
  geom_point( aes(shape = group), size = 2)

xyplot(concentration ~ date, data = MyData2, groups = location, type = c("p", "l"),
       auto.key = TRUE)

## without reshaping, you can plot 4 layers well manually, but the default labeling isn’t as nice

ggplot(data = MyData) +
  geom_line(aes(x = date, y = Well1, colour = "Well1")) +
  geom_line(aes(x = date, y = Well2, colour = "Well2")) +
  geom_point(aes(x = date, y = Well1, colour = "Well1", shape = group)) +
  geom_point(aes(x = date, y = Well2, colour = "Well2", shape = group))

Happy Holidays.  I hope one of these approaches will get you headed in the right direction.

—rjp

On Dec 24, 2015, at 7:51 AM, Steven Stoline <sstoline at gmail.com<mailto:sstoline at gmail.com>> wrote:

Dear All:

I am trying to plot two series in one graph. But I have some difficulties
to set up the y-axis lim. Also, the second series is not correctly graphed.

*Here is what I tried to do:*

### Define 2 vectors

Well1<-c(0.005,0.005,0.004,0.006,0.004,0.009,0.017,0.045,0.05,0.07,0.12,0.10,0.20,0.25)
Well2<-c(0.10,0.12,0.125,0.107,0.099,0.11,0.13,0.109,0.10,0.115,0.14,0.17,0.11)

### Calculate range from 0 to max value of Well1 and Well2
### g_range <- range(0, Well1, Well2)

max_y <- max(Well1, Well2)

### Graph Groundwater Concentrations using y axis that ranges from 0 to max
### value in Well1 or Well2 vector.  Turn off axes and
### annotations (axis labels) so we can specify them yourself

plot(Well1, type="o", pch=c(0,0,15,15,0,15,15,15,15,15,15,15,15,15),
col="blue", ylim=c(0,max_y), axes=FALSE, ann=FALSE, , lwd=3, cex=1.25)  ###
axes=FALSE,

### Make x axis using Jan 2005 - Dec 2008 labels

axis(1, at=1:14,
lab=c("Jan05","April05","Jul05","Oct05","Jan06","March06","Jun06","2Oct06","17Oct06","Jan07","April07","Jul07","Oct07","Dec07"))

*### Make y axis with horizontal labels , Here what I have the major
problem*

### I want the y-axis looks like: 0, 0.05, 0.10, 0.15, 20, 0.25

axis(2, las=0, at=6*0:max_y)  ### max_y

### Create box around plot

box()

### Graph Well2 with red dashed line and square points

### lines(Well2, type="o", pch=22, lty=2, col="red", lwd=3, cex=1.0)

lines(Well2, type="o", pch=c(0,15,15,15,0,15,15,15,0,15,15,15,15), lty=2,
col="red", lwd=3, cex=1.25)

### Create a title with a red, bold/italic font

title(main="Trichloroethene mg/L from Wells 1 and 2 - 2005-2007",
col.main="red", font.main=2)

### Label the x and y axes with dark green text

title(xlab="Time Points", col.lab=rgb(0,0.5,0))

title(ylab="Trichloroethene mg/L", col.lab=rgb(0,0.5,0))

### Create a legend

legend(1, g_range[2], c("Well1","Well2"), cex=1.0, col=c("blue","red"),
pch=15:15, lty=1:2);

with thanks
steve
-------------------------
Steven M. Stoline
1123 Forest Avenue
Portland, ME 04112
sstoline at gmail.com<mailto:sstoline at gmail.com>

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