Lots of clever ideas here, Alex, but I think you’re missing the point of what I 
ultimately want to do. I'm building a plotting program for which I want to plot 
any equation including those that have multiple values of y for a given x. An 
equation might branch at any point and might even have multiple branches both 
of which are unknown before plotting. 

Of course, there’s always the possibility that I’m not fully comprehending your 
suggestions??? What does NB stand for? Are you suggesting creating a new 
polygon every time a branch is detected? That just might work?!

Roger


> On Oct 30, 2020, at 4:12 PM, Alex Tweedly via use-livecode 
> <use-livecode@lists.runrev.com> wrote:
> 
> 
> On 30/10/2020 22:40, Roger Guay via use-livecode wrote:
>> Let’s try this again after spellchecking:
>> 
>> 
>> Yes, yours is a good example of a bifurcated line. But now imagine producing 
>> this line programmatically with an equation that:
>> 
>> Produces a constant y value of 149 as x progresses from 35 to 235 (no 
>> problem)
>> Then produces 2 different but simultaneous values of y as x progresses from 
>> 235 to 335. This is the problem as you don’t want the end point of the 
>> separated lines to connect. If you place an empty line in the points after 
>> each iteration beyond x = 235 then you end up with the bifurcated lines 
>> being points rather than a solid line.
>> 
>> How do plotting programs handle this situation????
> 
> 
> Here are two different ways you could do it.
> 
> 1. simple - assume there would (could) be two Y values for any x, and just 
> calculate two series, and combine them for output.
> 
> 2. harder - for each x value, keep track of the previous y value for each 
> series, and if necessary, put in a 'skip' plus new value plus skip ...
> 
> NB makes for a more complex polygon; each new x value after bifurcation 
> results in 5 lines added to the points.
> 
> on mouseup
>    local tSeries1, tSeries2, thepoints
>    if the shiftkey is down then
> 
>       -- the easy way - just allow for the possibility of two series of 
> points all along
>       repeat with i = 35 to 335
>          -- calculate series 1
>          if i < 235 then
>             put i,249 &CR after tSeries1
>          else
>             put i, 249+(i-235) &CR after tSeries1
>          end if
>          -- calculate series 2
>          if i < 235 then
>             -- do nothing - it's the same as series 1
>          else
>             put i, 249+2*(i-235) &CR after tSeries2
>          end if
>       end repeat
> 
>       put tSeries1 &CR & tSeries2 into thePoints
>       set the points of grc "X" to thePoints
>       set the foregroundColor of grc "X" to "blue"
>    else
>       -- the harder way - multiple series ...
>       -- does each step for each series ... much more complex polygon, but ...
>       local t1, t2, tLast1, tLast2
>       repeat with i = 35 to 335
>          -- calculate values
>          if i < 235 then
>             put 249 into t1
>             put 249 into t2
>          else
>             put 249+(i-235) into t1
>             put 249+2*(i-235) into t2
>          end if
>          -- put in series 1
>          if tLast1 is not empty then
>             put i-1, tLast1 &CR & i, t1 &CR after thePoints
>          end if
>          -- possibly put in series 2
>          if tLast1 <> tLast2 OR t1 <> t2 then
>             put CR after thePoints -- blank skip over to series 2 value
>             put i-1, tLast2 &CR & i, t2 &CR after thePoints
>             put CR after thePoints -- blank skip back to series 1
>          end if
>          put t1 into tLast1
>          put t2 into tLast2
>       end repeat
>       set the points of grc "X" to thePoints
>       set the foregroundColor of grc "X" to "red"
>    end if
> 
> end mouseup
> 
> 
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