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    <p>Below is some code <a href="http://www.mathworks.com/matlabcentral/fileexchange/27205" rel="nofollow noreferrer">ported from Matlab</a> to calculate the intersection between segments. If you apply it between the x axis (fixed) and each pair of successive points, you get a list of new coordinates that indicate the crossing points between your <code>geom_line</code> and the x axis. From this it's an easy step to shade the relevant polygons. Note that I haven't properly tested the ported Matlab code.</p> <p><img src="https://i.stack.imgur.com/PQzNz.png" alt="enter image description here"></p> <pre><code>## Ported from Matlab to R ## Copyright (c) 2010, U. Murat Erdem ## All rights reserved. ## http://www.mathworks.com/matlabcentral/fileexchange/27205 lineSegmentIntersect &lt;- function(XY1, XY2){ n_rows_1 &lt;- nrow(XY1) n_cols_1 &lt;- ncol(XY1) n_rows_2 &lt;- nrow(XY2) n_cols_2 &lt;- ncol(XY2) stopifnot(n_cols_1 == 4 &amp;&amp; n_cols_2 == 4) nc &lt;- n_rows_1 * n_rows_2 X1 &lt;- matrix(XY1[,1], nrow=nc, ncol=1) X2 &lt;- matrix(XY1[,3], nrow=nc, ncol=1) Y1 &lt;- matrix(XY1[,2], nrow=nc, ncol=1) Y2 &lt;- matrix(XY1[,4], nrow=nc, ncol=1) XY2 &lt;- t(XY2) X3 &lt;- matrix(XY2[1,], nrow=nc, ncol=1) X4 &lt;- matrix(XY2[3,], nrow=nc, ncol=1) Y3 &lt;- matrix(XY2[2,], nrow=nc, ncol=1) Y4 &lt;- matrix(XY2[4,], nrow=nc, ncol=1) X4_X3 &lt;- X4-X3 Y1_Y3 &lt;- Y1-Y3 Y4_Y3 &lt;- Y4-Y3 X1_X3 &lt;- X1-X3 X2_X1 &lt;- X2-X1 Y2_Y1 &lt;- Y2-Y1 numerator_a &lt;- X4_X3 * Y1_Y3 - Y4_Y3 * X1_X3 numerator_b &lt;- X2_X1 * Y1_Y3 - Y2_Y1 * X1_X3 denominator &lt;- Y4_Y3 * X2_X1 - X4_X3 * Y2_Y1 u_a &lt;- numerator_a / denominator u_b &lt;- numerator_b / denominator INT_X &lt;- X1 + X2_X1 * u_a INT_Y &lt;- Y1 + Y2_Y1 * u_a INT_B &lt;- (u_a &gt;= 0) &amp; (u_a &lt;= 1) &amp; (u_b &gt;= 0) &amp; (u_b &lt;= 1) PAR_B &lt;- denominator == 0 COINC_B &lt;- (numerator_a == 0 &amp; numerator_b == 0 &amp; PAR_B) data.frame(x=INT_X[INT_B], y=INT_Y[INT_B]) } set.seed(123) x &lt;- sort(runif(50, -10, 10)) y &lt;- jitter(sin(x), a=2) n &lt;- length(x) xy1 &lt;- matrix(c(-10, 0, 10, 0), ncol=4) xy2 &lt;- cbind(x[-n], y[-n], x[-1], y[-1]) test &lt;- lineSegmentIntersect(xy1, xy2) library(ggplot2) d &lt;- data.frame(x=x, y=y) d2 &lt;- rbind(d, test) d2 &lt;- subset(d2[order(d2$x), ], y &lt;=0) p &lt;- qplot(x, y, data=d, geom="path") p + geom_ribbon(data=d2, aes(ymin = 0, ymax = y), fill="red") </code></pre>
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    1. POShading an area between geom_line and the x-axis
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    1. USbaptiste
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    1. COI already figured out I would probably need to do something like that, hope to use this as an answer. Thanks a lot!
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      1. USGerrit Jan
    2. COHmm... one more question: I can work out the math to find the points at which the line segments intersect the x-axis, but I'll need some way to manipulate the x-values. Which is hard in this case, because I use the POSIXct class for the x-values. Is there any easy way to convert them to, for example, the number of seconds, so that I can do the calculation and afterwards convert back to POSIXct (and so find the correct date/time for the x-intersection)?
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      1. USGerrit Jan
    3. COsorry, no idea about POSIXct classes
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