Setting New Y Limit Dual Axis

I've been trying to figure out how to adjust the y min of a dual axis. A client requested that his current chart be adjusted. I found a post that touches on this here https://stackoverflow.com/questions/59293809/adjust-y-axis-limits-in-dual-y-axis-g

Trying to follow the example two things are occurring, 1) the distance between where the bar is set and the edge of the plot have a gap 2) the chart data itself has changed.

> structure(stats)
# A tibble: 17 × 3
`Snapshot Date` avg_asking_rate availability_rate
<date>                    <dbl>             <dbl>
1 2019-03-31                 6.27             10.1 
2 2019-06-30                 6.30             11.2 
3 2019-09-30                 6.54             11.0 
4 2019-12-31                 7.33             11.0 
5 2020-03-31                 7.00              8.56
6 2020-06-30                 7.64              9.56
7 2020-09-30                 6.98             10.1 
8 2020-12-31                 7.16              9.46
9 2021-03-31                 7.39              9.36
10 2021-06-30                 7.37              8.60
11 2021-09-30                 6.88              8.29
12 2021-12-31                 7.18              6.81
13 2022-03-31                 7.05              5.73
14 2022-06-30                 7.39              5.79
15 2022-09-30                 7.45              7.04
16 2022-12-31                 7.68              7.36
17 2023-03-31                 8.00              7.52
ymin = min(stats$avg_asking_rate)
stats %>%      
ggplot(., aes(x = `Snapshot Date`))+
ggtitle("Rent ($/SF NNN)       (%)") +                                                                                                                        
geom_tile(aes(y = avg_asking_rate/2 + ymin, height = avg_asking_rate, width = 70, fill = "Avg. Asking Rate")) + 
geom_line(aes(y = availability_rate, colour = "Availability Rate"),
size = 1)+
scale_x_date(name = NULL,
breaks = quarter_labels[, "Snapshot Date", drop=T],
labels = quarter_labels[, "qlabel", drop=T])+
scale_y_continuous(name = NULL,
sec.axis = sec_axis(~.-ymin),
breaks = scales::breaks_pretty(n = 5))) +
scale_fill_manual(name = NULL, values = c("Avg. Asking Rate" = secondary_pal[5], "Availability Rate" = primary_pal[4]),guide = guide_legend(order = 1))+
scale_colour_manual(name = NULL, values = c("Availability Rate" = primary_pal[4]), guide = guide_legend())

I just want it to look like my original plot, but with the primary minimum starting at 0. At my wits end here. If you have to tear my code apart please feel free to do so.

The Original:

If I'm understanding the problems, this is how I would do it. First, make the data:

stats <- tibble::tribble(
  ~`Snapshot Date`, ~avg_asking_rate, ~availability_rate,
"2019-03-31",                 6.27,             10.1 ,
"2019-06-30",                 6.30,             11.2 ,
"2019-09-30",                 6.54,             11.0 ,
"2019-12-31",                 7.33,             11.0 ,
"2020-03-31",                 7.00,              8.56,
"2020-06-30",                 7.64,              9.56,
"2020-09-30",                 6.98,             10.1 ,
"2020-12-31",                 7.16,              9.46,
"2021-03-31",                 7.39,              9.36,
"2021-06-30",                 7.37,              8.60,
"2021-09-30",                 6.88,              8.29,
"2021-12-31",                 7.18,              6.81,
"2022-03-31",                 7.05,              5.73,
"2022-06-30",                 7.39,              5.79,
"2022-09-30",                 7.45,              7.04,
"2022-12-31",                 7.68,              7.36,
"2023-03-31",                 8.00,              7.52)
stats$`Snapshot Date` <- lubridate::ymd(stats$`Snapshot Date`)

The quarter_labels object wasn't defined, so I just defined them in the data.

stats$quarter_labels <- trimws(paste(c("2019", "","","", "2020", "", "","", "2021", "","","", "2022", "","","","2023"), 
                              c(rep(c("Q1", "Q2", "Q3", "Q4"), 3), "Q1"), sep=" "))

Next, get the maximum values on the primary and secondary axis variables:

aar_max <- max(stats$avg_asking_rate)
ar_max <- max(stats$availability_rate)

You can plot the primary axis on its original scale and the secondary axis needs the linear transformation 0 + y*(aar_max/ar_max), which you can implement simply by multiplying the availability_rate variable by aar_max/ar_max in the line aesthetic. I would use geom_bar() instead of geom_tile() for the primary axis variable - it seems a bit easier and you'll automatically get the primary axis going to zero. You can unwind the transformation on the secondary axis by using the linear transformation 0 + .*(ar_max/aar_max). This will put the secondary axis values on the right scale. Finally, in scale_y_manual() you can set limits = c(0,8.1) and expand=c(0,0) to force the limits to be what you set without extra padding on the top and bottom. You may have to fiddle with the upper limit a bit to make sure it's where you want.

library(ggplot2)
ggplot(stats, aes(x = `Snapshot Date`))+
  ggtitle("Rent ($/SF NNN)       (%)") +   
  geom_bar(aes(y=avg_asking_rate, fill="Avg. Asking Rate"), stat="identity") +
  geom_line(aes(y = availability_rate*(aar_max/ar_max), colour = "Availability Rate"),
            size = 1)+
  scale_x_date(name = NULL,
               breaks = stats$`Snapshot Date`,
               labels = stats$quarter_labels)+
  scale_y_continuous(name = NULL,
                     limits = c(0,8.1), 
                     expand = c(0,0), 
                     sec.axis = sec_axis(~0 + .*(ar_max/aar_max)),
                     breaks = scales::breaks_pretty(n = 5)) +
  scale_fill_manual(name = NULL, values = c("Avg. Asking Rate" = "gray", "Availability Rate" = "black"),guide = guide_legend(order = 1))+
  scale_colour_manual(name = NULL, values = c("Availability Rate" = "black"), guide = guide_legend()) + 
  theme_minimal() + 
  theme(axis.text.x = element_text(angle=90, hjust=1), 
        panel.grid.major.x = element_blank(),
        panel.grid.minor.x = element_blank())

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^{Created on 2023-08-09 with reprex v2.0.2}