diff --git a/R/theme_heatmap.R b/R/theme_heatmap.R
index d66325b..0eac815 100644
--- a/R/theme_heatmap.R
+++ b/R/theme_heatmap.R
@@ -1,5 +1,5 @@
-theme_heatmap<-function(x){
-  list(geom_tile(),
+theme_heatmap<-function(x,line.color="#FFFFFF00"){
+  list(geom_tile(color=line.color),
        scale_fill_gradientn(colors=col2(200)),
        theme(axis.text.x = element_text(angle = 90, hjust = 1, vjust=0.5),
              panel.background = element_blank(),