I'm trying to plot a line graph where the colors change depending on the value of a different variable (e.g. y > 3 makes it red, otherwise green). However, when doing this with a line size greater than 1, the individual segments making up the line become very obvious, which is not the case when having a single color for the line. This especially becomes a problem when plotting e.g. daily data for several years, which is what I have in my real dataset. Here is a reproducible example:

```
es_sub <- structure(list(date = structure(c(15323, 15324, 15327, 15328, 
15329, 15330, 15331, 15334, 15335, 15336, 15337, 15338, 15341, 
15342, 15343, 15344, 15345, 15348, 15349, 15350, 15351, 15352, 
15355, 15356, 15357, 15358, 15359, 15362, 15363, 15364, 15365, 
15366, 15369, 15370, 15371, 15372, 15373, 15376, 15377, 15378, 
15379, 15380, 15383, 15384, 15385, 15386, 15387, 15390, 15391, 
15392, 15393), class = "Date"), yield = c(4.3923, 4.2818, 4.22, 
4.15263, 4.07348, 3.97288, 3.86626, 3.77447, 3.71084, 3.63926, 
3.59491, 3.58036, 3.57425, 3.57158, 3.56352, 3.56215, 3.57652, 
3.57602, 3.56521, 3.541, 3.50181, 3.47117, 3.43809, 3.39826, 
3.35215, 3.3009, 3.23324, 3.16079, 3.09816, 3.05649, 3.00415, 
2.93571, 2.87404, 2.81528, 2.7418, 2.65998, 2.57848, 2.51727, 
2.46631, 2.43695, 2.43333, 2.45101, 2.46015, 2.47799, 2.51173, 
2.54209, 2.56716, 2.59079, 2.60407, 2.60738, 2.60619), col = structure(c(1L, 
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 
2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 
2L, 2L), .Label = c("2", "3"), class = "factor")), .Names = c("date", 
"yield", "col"), row.names = 500:550, class = "data.frame")

ggplot(aes(x=date, y=yield, group=1), data=es_sub) + 
geom_line(aes(color=ifelse(es_sub$yield>3,"red","green")), size=3) +
theme(legend.position="none")

ggplot(aes(x=date, y=yield, group=1), data=es_sub) + geom_line(size=3)

```

The individual segments are obvious in the first graph, but not the second. Is there a way around this? 

UPDATE: I tried what Joran suggested (i.e. mapping group in the same way as color), but that causes this problem instead (see chart) (this is my real plot. It has about 1000 observations, and I didn't want to put them all in the reproducible example).

```
[1]: http://i.stack.imgur.com/38KPB.png

```