How to make the speed profile of a moving object? - r

Question

I am an R beginner user and I face the following problem. I have the following data frame:
distance speed
1 61.0 36.4
2 51.4 35.3
3 42.2 34.2
4 33.4 32.8
5 24.9 31.3
6 17.5 28.4
7 11.5 24.1
8 7.1 19.4
9 3.3 16.9
10 0.5 15.5
11 4.4 15.1
12 8.5 15.5
13 13.1 17.3
14 18.8 20.5
15 25.7 24.1
16 33.3 26.3
17 41.0 27.0
18 48.7 27.7
19 56.6 28.4
20 64.8 29.2
21 73.6 31.7
22 83.3 34.2
23 93.4 35.3
The column distance represents the distance of a following object over a specific point and the column speed the object's speed. As you can see the object is getting closer to the point and then it is getting away. I am trying to make its speed profile. I tried the following code but it didn't give me the plot I want (because I want to show how its speed is changing when the moving object moves closer and past the reference point)
ggplot(speedprofile, aes(x = distance, y = speed)) + #speedprofile is the data frame
geom_line(color = "red") +
geom_smooth() +
geom_vline(xintercept = 0) # the vline is the reference line
The plot is the following:
Then, I tried to set the first 10 distances as negative manually which are prior to zero (0). So I get a plot closer to that I want:
But there is a problem. The distance can't be defined as negative.
To sum up, the expected plot is the following (and I am sorry for the quality).
Do you have any ideas on how to solve this?
Thank you in advance!

Answer 1

You can do something like this to auto-compute the change point (to know when the distance should be negative) and then set the axis labels to be positive.
Your data (in case anyone needs it to answer):
read.table(text="distance speed
61.0 36.4
51.4 35.3
42.2 34.2
33.4 32.8
24.9 31.3
17.5 28.4
11.5 24.1
7.1 19.4
3.3 16.9
0.5 15.5
4.4 15.1
8.5 15.5
13.1 17.3
18.8 20.5
25.7 24.1
33.3 26.3
41.0 27.0
48.7 27.7
56.6 28.4
64.8 29.2
73.6 31.7
83.3 34.2
93.4 35.3", stringsAsFactors=FALSE, header=TRUE) -> speed_profile
Now, compute the "real" distance (negative for approaching, positive for receding):
speed_profile$real_distance <- c(-1, sign(diff(speed_profile$distance))) * speed_profile$distance
Now, compute the X axis breaks ahead of time:
breaks <- scales::pretty_breaks(10)(range(speed_profile$real_distance))
ggplot(speed_profile, aes(real_distance, speed)) +
geom_smooth(linetype = "dashed") +
geom_line(color = "#cb181d", size = 1) +
scale_x_continuous(
name = "distance",
breaks = breaks,
labels = abs(breaks) # make all the labels for the axis positive
)
Provided fonts are working well on your system you could even do:
labels <- abs(breaks)
labels[(!breaks == 0)] <- sprintf("%s\n→", labels[(!breaks == 0)])
ggplot(speed_profile, aes(real_distance, speed)) +
geom_smooth(linetype = "dashed") +
geom_line(color = "#cb181d", size = 1) +
scale_x_continuous(
name = "distance",
breaks = breaks,
labels = labels,
)