R中的ggplot - 将x轴分成两组

Question

I would like to divide my x axis into two groups. I managed already to make one group using facet but can't make a second one. Here is a picture of what I already have and what I want additionally. Here is also my command:

ggplot(myk.p, aes(x=Group, y=Colonization, fill=MO)) +
    geom_bar(stat="identity", colour="black") +
    scale_fill_manual(values=c("#000000","#7a7a7a","#c0c0c0")) +
    theme(panel.margin = unit(0, "lines"), strip.background =
    element_blank()) + facet_wrap(~Water, switch = "x", scales = "free_x") +
    ggtitle("Root mycorrhization in % (Populus nigra)")

Example Data.csv:

"Group" "Colonization"  "MO"    "Water" "Fertilizer"
"P1"    69,2307692308   "C" "once"  "without"
"P2"    71,8232044199   "F" "once"  "without"
"P3"    82,5174825175   "I" "once"  "without"
"P4"    66,4122137405   "C" "once"  "with"
"P5"    86,013986014    "F" "once"  "with"
"P6"    98,7654320988   "I" "once"  "with"
"P7"    41,9540229885   "C" "twice" "without"
"P8"    65,7894736842   "F" "twice" "without"
"P9"    63,9705882353   "I" "twice" "without"
"P10"   62,5850340136   "C" "twice" "with"
"P11"   87,012987013    "F" "twice" "with"
"P12"   89,696969697    "I" "twice" "with"

Answer 1

Thanks for the comments. aosmith: Works perfect. The only problem I now have is that the ordering is wrong. I want to start it from P1,P2,...P12 This is what I got now:

almost