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The problem is a bit tricky, I will try to be as clear as possible. I have several lists (let's call them x, y, z and k), all of the same length, and where the i-th value of each list corresponds to a single observation (so for instance x[[3]], y[[3]], z[[3]] and k[[3]] are the values of four variables for the third observation). What I want to do is a scatter plot, where the x-axis reports the values of x and the y-axis the values of y. Now, for each pair (x[[i]], y[[i]]) I want a red dot if z[[i]] > k[[i]], and a black dot otherwise. Additional issue: for the first observations, the list k is empty, so I want to plot a red dot for such observations. Any idea on how is it possible to do it?

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  • 1
    $\begingroup$ Hint: You can style individual points in ListPlot using Style[{x, y}, color]. $\endgroup$
    – C. E.
    May 27 '20 at 14:16
  • $\begingroup$ That is very helpful, I will have a try with that. Thank you. $\endgroup$ May 27 '20 at 14:19
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Here is an example with made up data:

Clear[y, z, k]
x = Range[20];
{y, z, k} = RandomInteger[{-10, 10}, {3, 20}];

ListPlot[
  MapThread[Style[{#1, #2}, If[#3 > #4, Red, Black]] &, {x, y, z, k}],
  PlotRange -> All,
  PlotStyle -> PointSize[0.02]
]

formatted points


For your second part ("Additional issue: for the first observations, the list k is empty, so I want to plot a red dot for such observations."), you should really provide a sample of your data. Any answer would depend on its precise structure.

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  • $\begingroup$ That is perfect, thank you! $\endgroup$ May 27 '20 at 14:30
  • $\begingroup$ For the last part you could just PadLeft the kkkkk... list with some non-number element e.g {} to the same size as any of the the xxxxx... or yyyyy.... zzzzz... lists before transposing to {{x,y,z,{}},{x,y,z,{}},{x,y,z,k},{x,y,z,k}... etc}. Then change your style function to only apply if #4 is not {}. $\endgroup$
    – flinty
    May 27 '20 at 14:33
  • $\begingroup$ Thank you for the added comment, your code actually worked nice as it is :) $\endgroup$ May 27 '20 at 19:57
  • $\begingroup$ I have an additional question. What if I would like to use different shapes instead of different colours? $\endgroup$ Jun 5 '20 at 9:21
  • $\begingroup$ @MarcoB can you help me? $\endgroup$ Jun 7 '20 at 12:11
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Generate random data with first two values of k missing.

SeedRandom[1234]

{x, y, z, k} = 
 ReplacePart[RandomReal[{0, 10}, {4, 20}], {{4, 1}, {4, 2}} -> Missing]

Just use Graphics primitives to make the scatter plot.

Legended[Graphics[{AbsolutePointSize[
    4], {If[#[[4]] === Missing || #[[3]] > #[[4]], Red, Black], 
      Point[#[[1 ;; 2]]]} & /@ Transpose[{x, y, z, k}]},
  Frame -> True,
  FrameLabel -> (Style[#, 14, Bold, Italic] & /@
      {"x", "y"} /. 
     s_String :> Subscript[s, "i"]),
  AspectRatio -> 1/GoldenRatio],
 PointLegend[{Red, Black}, {"z" > "k", "z" <= "k"} /. 
   s_String :> Subscript[s, "i"]]]

enter image description here

EDIT: Alternatively, you get the same plot using GatherBy with the criteria to generate two data sets.

plotData = #[[All, 1 ;; 2]] & /@ 
   GatherBy[Transpose[{x, y, z, k}], #[[4]] === Missing || #[[3]] > #[[4]] &];

ListPlot[plotData,
 PlotStyle -> {Red, Black},
 Frame -> True,
 FrameLabel -> (Style[#, 14, Bold, Italic] & /@
     {"x", "y"} /. 
    s_String :> Subscript[s, "i"]),
 PlotLegends -> {"z" > "k", "z" <= "k"} /.
  s_String :> Subscript[s, "i"]]
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  • $\begingroup$ I am sorry, I just notice I did not answer you. Thank you for your answer. I just have an additional question: what if I would like to do the same, but using different shapes instead of different colours? $\endgroup$ Jun 7 '20 at 12:12
  • $\begingroup$ Use the option PlotMarkers, e.g., PlotMarkers -> Automatic $\endgroup$
    – Bob Hanlon
    Jun 7 '20 at 15:24
  • $\begingroup$ Thank you! I cannot believe it was that easy, I was trying much more complicated stuff. $\endgroup$ Jun 9 '20 at 7:41

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