title: Arrange x-axis in ascending order
body: <p>I am trying to plot in a stacked bar chart the accumulative cost of seven different units, using ggplot2 and geom_col. The challenge I have is when I plot the data, in the x-axis I do not get the units in an ascending order.</p>
  
  <pre><code>&gt; b1
    Unit variable   value
    1   60k      BOM 2950806
    2  100k      BOM 3236021
    3  120k      BOM 3533470
    4  140k      BOM 3611764
    5  170k      BOM 3855279
    6  200k      BOM 4166095
    7  230k      BOM 4468843
    8   60k     NaOH  255676
    9  100k     NaOH  255676
    10 120k     NaOH  255676
    11 140k     NaOH  255676
    12 170k     NaOH  255676
    13 200k     NaOH  255676
    14 230k     NaOH  255676
  library(ggplot2)
  p1 &lt;- ggplot(b1, aes(Unit, value, fill = variable))+
  geom_col(position = "stack", colour = "black")+
  labs(x = expression("Unit size"),
     y = expression("Cost"*" / [NOK]"))+
  theme_bw()+
  theme(plot.background = element_blank(),
      panel.grid.major = element_blank(),
      panel.grid.minor = element_blank())
  p1
  </code></pre>
  
  <p>I expect the x-axis to show  60k, 100k, 120k, 140k, 200k, 230k, but instead it shows</p>
  
  <p>100k, 120k, 140k, 200k, 230k, 60k.</p>