objective c - Changing the position of a CorePlot annotation after zooming(iOS) -

i using coreplot scatterplot display data readings external bluetooth sensor. ive gone through following tutorial: http://www.raywenderlich.com/13271/how-to-draw-graphs-with-core-plot-part-2 , ive got working way 1 exception. when user taps on 1 of data points, annotation placed on top of data point.

here how create , add annotations:

nsnumber *value = [self numberforplot:plot field:cptscatterplotfieldy recordindex:idx]; if (!valueannotation) {     nsnumber *x = [nsnumber numberwithint:0];     nsnumber *y = [nsnumber numberwithint:0];     nsarray *anchorpoint = [nsarray arraywithobjects:x, y, nil];     valueannotation = [[cptplotspaceannotation alloc] initwithplotspace:plot.plotspace anchorplotpoint:anchorpoint]; }  // 5 - create text layer annotation nsstring *stringvalue = [nsstring stringwithformat:@"%@", value];  cpttextlayer *textlayer = [[cpttextlayer alloc] initwithtext:stringvalue style:style]; valueannotation.contentlayer = textlayer;  // 7 - anchor point annotation cgfloat x = (cgfloat)idx; nsnumber *anchorx = [nsnumber numberwithfloat:x]; cgfloat y = [value floatvalue]; //edit line? nsnumber *anchory = [nsnumber numberwithfloat:y]; valueannotation.anchorplotpoint = [nsarray arraywithobjects:anchorx, anchory, nil];  // 8 - add annotation [plot.graph.plotareaframe.plotarea addannotation:valueannotation]; 

ive tried adding small value anchory move annotation above plot point:

cgfloat y = [value floatvalue] + 5; 

although works great @ first, when user zooms in graph annotation moves further , further away plot point. how can keep annotation above or below plot point while zooming?

i thought might have implement cptplotspacedelegate method plotspace: shouldscaleby: aboutpoint: method, , im stuck. have:

-(bool)plotspace:(cptplotspace *)space shouldscaleby:(cgfloat)interactionscale aboutpoint:(cgpoint)interactionpoint {      cptgraph *graph = self.hostview.hostedgraph;      nsarray *annotations = graph.plotareaframe.plotarea.annotations;       if (annotations.count > 0) {          cptplotspaceannotation *annotation = [annotations objectatindex:0];           nsnumber *anchorx = /* ? */;          nsnumber *anchory = /* ? */;           annotation.anchorplotpoint = [nsarray arraywithobjects:anchorx, anchory, nil];      }      return yes;  } 

im not sure go here. or direction provide appreciated.

use displacement property offset annotation away anchor point. displacement measured in pixels won't change plot space.