We will draw a regression line through some points, as in http://pgfplots.net/tikz/examples/regression-line/ . This time, we will implement a style for negative and positive residuals using an optional argument.
Edit and compile if you like:\documentclass[border=5pt]{standalone}\usepackage{pgfplots}\pgfplotsset{width=7cm,compat=1.8}\usepackage{pgfplotstable}\usepackage{helvet}\usepackage[eulergreek]{sansmath}\pgfmathsetseed{10} % set the random seed\pgfplotstableset{ % Define the equations for x and ycreate on use/x/.style={create col/expr={42+2*\pgfplotstablerow}},create on use/y/.style={create col/expr={(0.6*\thisrow{x}+130)+8*rand}}}% create a new table with 30 rows and columns x and y:\pgfplotstablenew[columns={x,y}]{30}\loadedtable% Calculate the regression line\pgfplotstablecreatecol[linear regression]{regression}{\loadedtable}\pgfplotsset{colored residuals/.style 2 args={only marks,scatter,point meta=explicit,colormap={redblue}{color=(#1) color=(#2)},error bars/y dir=minus,error bars/y explicit,error bars/draw error bar/.code 2 args={\pgfkeys{/pgf/fpu=true}\pgfmathtruncatemacro\positiveresidual{\pgfplotspointmeta<0}\pgfkeys{/pgf/fpu=false}\ifnum\positiveresidual=0\draw [#2] ##1 -- ##2;\else\draw [#1] ##1 -- ##2;\fi},/pgfplots/table/.cd,meta expr=(\thisrow{y}-\thisrow{regression})/abs(\thisrow{y}-\thisrow{regression}),y error expr=\thisrow{y}-\thisrow{regression}},colored residuals/.default={red}{blue}}\begin{document}\begin{tikzpicture}[every node/.style={font=\sffamily}]\begin{axis}[xlabel=Weight (kg), % label x axisylabel=Height (cm), % label y axisaxis lines=left, %set the position of the axesxmin=40, xmax=105, % set the min and max values of the x-axisymin=150, ymax=200, % set the min and max values of the y-axistick label style = {font=\sansmath\sffamily},every axis label = {font=\sansmath\sffamily},legend style = {font=\sansmath\sffamily},label style = {font=\sansmath\sffamily},]\makeatletter\addplot [colored residuals] table {\loadedtable};\addplot [no markers,thick, black] table [y=regression] {\loadedtable} ;\end{axis}\end{tikzpicture}\end{document}