title: ggplot2: Plotting regression lines with different intercepts but with same slope
body: <p>I want to plot regression lines with different intercepts but with the same slope. </p>
  
  <p>With the following <code>ggplot2</code> code, I can plot regression lines with different intercepts and different slopes. But could not figured out how to draw regression lines with different different intercepts but the same slopes.</p>
  
  <pre><code>library(ggplot2)
      ggplot(data=df3, mapping=aes(x=Income, y=Consumption, color=Gender)) + geom_point() + 
      geom_smooth(data=df3, method = "lm", se=FALSE, mapping=aes(x=Income, y=Consumption))
  
  Consumption &lt;- c(51, 52, 53, 54, 56, 57, 55, 56, 58, 59, 62, 63)
  Gender &lt;- gl(n = 2, k = 6, length = 2*6, labels = c("Male", "Female"), ordered = FALSE)
  Income &lt;- rep(x=c(80, 90, 100), each=2)
  df3 &lt;- data.frame(Consumption, Gender, Income)
  df3
  
  # Regression with same slope but different intercepts for each Gender
  fm1 &lt;- lm(formula=Consumption~Gender+Income, data=df3)
  summary(fm1)
  
  Call:
  lm(formula = Consumption ~ Gender + Income, data = df3)
  
  Residuals:
      Min      1Q  Median      3Q     Max 
  -0.8333 -0.8333  0.1667  0.1667  1.1667 
  
  Coefficients:
               Estimate Std. Error t value Pr(&gt;|t|)    
  (Intercept)  26.83333    2.54557   10.54 2.30e-06 ***
  GenderFemale  5.00000    0.45812   10.91 1.72e-06 ***
  Income        0.30000    0.02805   10.69 2.04e-06 ***
  ---
  Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
  
  Residual standard error: 0.7935 on 9 degrees of freedom
  Multiple R-squared:  0.9629,    Adjusted R-squared:  0.9546 
  F-statistic: 116.7 on 2 and 9 DF,  p-value: 3.657e-07
  </code></pre>