我刚开始尝试通过使用 R 中的 ggplot2 包来可视化我的数据，最后到了一个点，几乎一切看起来都很好。意思是，轴标记正确，具有正确的刻度线等。
现在的问题是，虽然我在运行下面的代码时没有收到错误消息，但图中的实际图形没有显示出来。这让我很难弄清楚到底出了什么问题。任何关于为什么会（不）发生的建议将不胜感激。
time <- seq(as.Date("2010-04-01"), as.Date("2012-02-01"), by = "2 months")
日期向量，用作刻度线。每 2 个月标记一次，从数据中最早的日期开始，直到最后一个日期之后。
qplot(date, sums/1000, data=sums[(sums$lake=="Gollinsee") & (sums$maize=="no maize"),] ,color=location, group=location, main="Gollinsee without maize") +
geom_line(size=0.7) + geom_point(size=3) +
labs(x="Date", y=expression("Total C [" * g %.% m^-3 * "]")) +
scale_x_date(breaks=(time), limits=c(min(time), max(time)), labels=format.Date(time, "%b %Y"))
一切都按预期运行，直到我添加最后一行。没有它，x 轴太杂乱，日期的格式也不理想。好了，生成了以下图像（不幸的是，我还不允许发布图像，但链接应该可以工作）：
http://i.stack.imgur.com/I6rRY.png
在没有修改 x 轴的情况下，情节看起来不太像样。
http://i.stack.imgur.com/WidpZ.png
亲切的问候！
编辑： dput(sums) 的输出
structure(list(date = structure(c(1L, 1L, 2L, 2L, 3L, 3L, 4L,
5L, 6L, 6L, 7L, 7L, 8L, 8L, 9L, 9L, 11L, 11L, 19L, 10L, 10L,
12L, 12L, 14L, 14L, 16L, 16L, 19L, 15L, 20L, 20L, 13L, 13L, 17L,
17L, 18L, 20L, 20L, 22L, 22L, 18L, 4L, 21L, 25L, 28L, 25L, 25L,
23L, 23L, 23L, 23L, 28L, 24L, 24L, 24L, 24L, 22L, 22L, 22L, 22L,
21L, 25L, 35L, 35L, 37L, 37L, 35L, 35L, 37L, 37L, 39L, 39L, 39L,
39L, 36L, 28L, 33L, 33L, 33L, 33L, 31L, 31L, 31L, 31L, 29L, 29L,
29L, 26L, 26L, 26L, 26L, 27L, 27L, 27L, 27L, 30L, 30L, 30L, 30L,
32L, 32L, 32L, 32L, 34L, 34L, 34L, 34L, 38L, 38L, 38L, 38L, 36L,
36L), .Label = c("2010-04-06", "2010-04-08", "2010-05-04", "2010-05-06",
"2010-05-19", "2010-06-01", "2010-06-03", "2010-06-29", "2010-06-30",
"2010-07-27", "2010-07-29", "2010-08-24", "2010-08-26", "2010-09-21",
"2010-09-23", "2010-10-19", "2010-10-21", "2010-11-16", "2010-11-18",
"2011-01-05", "2011-02-24", "2011-03-22", "2011-04-05", "2011-04-07",
"2011-05-03", "2011-05-05", "2011-05-31", "2011-06-01", "2011-06-28",
"2011-07-01", "2011-07-26", "2011-07-28", "2011-08-23", "2011-08-25",
"2011-09-20", "2011-09-22", "2011-10-17", "2011-10-20", "2012-01-10"
), class = "factor"), lake = structure(c(1L, 1L, 2L, 2L, 1L,
1L, 2L, 1L, 1L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 1L, 1L,
1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 1L, 1L,
1L, 1L, 1L, 1L, 2L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L,
2L, 2L, 2L, 2L, 2L, 2L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
1L, 1L, 1L, 1L, 1L, 2L, 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, 2L, 2L, 2L, 2L, 2L, 2L), .Label = c("Gollinsee",
"Schulzensee"), class = "factor"), location = structure(c(2L,
1L, 2L, 1L, 2L, 1L, 2L, 2L, 2L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 2L,
1L, 1L, 2L, 1L, 2L, 1L, 2L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 1L,
1L, 2L, 2L, 2L, 2L, 2L, 1L, 1L, 1L, 2L, 2L, 1L, 1L, 1L, 1L, 1L,
2L, 2L, 2L, 1L, 1L, 2L, 2L, 2L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L,
2L, 1L, 1L, 1L, 1L, 2L, 1L, 2L, 1L, 2L, 2L, 2L, 1L, 1L, 2L, 2L,
2L, 1L, 1L, 2L, 2L, 1L, 2L, 2L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L,
1L, 1L, 2L, 1L, 2L, 1L, 2L, 2L, 1L, 1L, 2L, 1L, 2L, 1L, 1L, 2L
), .Label = c("littoral", "pelagic"), class = "factor"), maize = structure(c(2L,
2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L,
2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 1L, 2L, 2L,
2L, 2L, 2L, 2L, 1L, 1L, 1L, 2L, 2L, 1L, 1L, 1L, 1L, 2L, 1L, 2L,
1L, 2L, 2L, 1L, 2L, 1L, 2L, 2L, 1L, 1L, 2L, 2L, 2L, 2L, 1L, 1L,
2L, 2L, 1L, 1L, 2L, 2L, 2L, 1L, 1L, 2L, 1L, 1L, 1L, 2L, 2L, 1L,
2L, 1L, 2L, 2L, 1L, 1L, 2L, 1L, 1L, 2L, 2L, 1L, 2L, 1L, 1L, 2L,
2L, 1L, 1L, 1L, 2L, 2L, 1L, 2L, 2L, 1L, 2L, 1L, 1L, 2L, 2L, 1L
), .Label = c("maize", "no maize"), class = "factor"), sums = c(388.7,
254.5, 320.4, 373.3, 224.1, 182.4, 245.5, 258.7, 47.7, 60.5,
58.3, 103.7, 311.9, 207.7, 283.9, 308.3, 237.9, 165.4, 556.4,
1090.4, 1792.6, 1350.5, 932.4, 526.5, 1157.8, 2343.7, 4570.9,
393.2, 315.5, 351, 1008, 504.8, 289.9, 113.4, 117.7, 707.9, 193.4,
122.2, 828.2, 1106.4, 837.8, 148, 777.6, 1824.6, 388, 1658.6,
636.3, 1434.7, 770.8, 1776.4, 705.9, 1634.5, 1237.1, 1882.5,
1073.8, 2283, 1030.7, 273, 194.6, 788.6, 1034.9, 420.7, 404.6,
295.9, 332, 229.3, 381, 304.2, 780.2, 227.3, 50.6, 509.3, 209.7,
675.8, 691.9, 651.7, 635.1, 560.8, 602.2, 1243.1, 733.8, 669.1,
574.5, 379.6, 1405.3, 513.6, 612.4, 1396.4, 256.7, 486.8, 521.3,
55.5, 48.2, 181.7, 281.3, 322.7, 478.1, 208.4, 173.8, 599.2,
262.9, 760.7, 365.5, 746.6, 790.3, 581.7, 388.9, 826.4, 132.6,
167.5, 817.6, 425.1, 582.2)), .Names = c("date", "lake", "location",
"maize", "sums"), class = "data.frame", row.names = c(NA, -113L))
亲切的问候！