title: How to assign negative and positive effects different color in forestplot?
body: <p>I have made a forest plot and would like to make the square with whiskers red for all the estimates that are negative and all the positive should remain blue. 
  Is this possible?</p>
  
  <pre><code>tabletext&lt;-cbind(
    c("","Intervention","1",  "2", "3", 
      "4", "5", "6", "7", 
      "Baseline (ref)"),
    c("","Predicted 95% CI","151  [128;175]", "146 [128;163]",
      "161 [144;179]","147 [130;164]", "137 [121;154]","141 [126;156]",
      "180[156;205 ]", "146 [129;162]"),
    c("","p-value", ".98",".99", ".060", 
      ".99", ".61",".92",".0016","ref"))
  data &lt;- data.frame(coef=c(NA,NA,6.093, 0.060,16.01,1.142,-8.42,-4.58,     34.45,0),
  low=c(NA,NA,-12.4,-9.02,3.55,-9.82,-20.65,-15.16,15.97,0),     high=c(NA,NA,24.62,9.14,28.464,12.11,3.80, 5.99, 52.92,0))
  
  
  forest&lt;-forestplot(tabletext,
                         upper = data$high,
                         lower=data$low,
                         mean = data$coef,
                         zero = 0,
                         boxsize = 0.2,
                         cex=0.9,
                         lineheight = "auto",
                         title="TITLE",
                         xlab = "Effect size 95% CI",
                         vertices = TRUE,
                         col=fpColors(box="darkblue", lines="darkblue", zero = "gray50"),
                         txt_gp=fpTxtGp(label=gpar(cex=1),
                                        ticks=gpar(cex=1),
                                        xlab=gpar(cex = 1),
                                        title=gpar(cex = 1)))
  </code></pre>
  
  <p>So, in this case, the square and whiskers of factor 5+6 should be red. The rest of the plot should remain the same.</p>