Graphing in Fortran, Algorithm Assistance Needed - algorithm

Question

I am trying to create a fortran program that will plot X,Y coordinates to the screen. I have the program receive plots from an input file. in the input file the first entry is the amount of points. every line after that has an x value then a space then the corresponding y values. I then have the program bubble sort the x values from smallest to greatest. next i want to graph these values. i want to graph them so that the scale of the x axis is xmax-xmin/#of points and the y axis is the same. how would i go about printing a '*' for points as a dirty graph??? im not looking for the entire answer just some direction of how to form the algorithm for the write. Listed below is the code that i have so far and a sample input file
Input File
10
-4 16
-3 9
-2 4
-1 1
0 0
1 1
2 4
3 9
4 16
5 25
Code
*START OF CODE
PROGRAM MAIN
*DECLARATIONS
INTEGER EXISTS, DONE, POINTS, USED, I, E
REAL XSTEP, YSTEP, XMIN, XMAX, YMIN, YMAX
REAL X(100), Y(100), A, B
CHARACTER INNAME*30, OUTNAME*30
LOGICAL MORE
CHARACTER (70) LINE
*INITIALIZATIONS
INNAME='NULL.DOC'
EXISTS=0
DONE=0
POINTS=0
USED=0
I=0
E=0
MORE = .TRUE.
A=0
B=0
XSTEP = 0
YSTEP = 0
XMIN=0
XMAX=0
YMIN=0
YMAX=0
*CLEAR THE ARRAYS
DO I=1,100
X(I)=0
Y(I)=0
END DO
I=0
*FORMATS
5 FORMAT(' ','POINT #', I3, ' X=', F5.2,' Y=', F5.2)
10 FORMAT(' ','XMIN=',F5.2, ' XMAX=', F5.2)
15 FORMAT(' ','YMIN=',F5.2, ' YMAX=', F5.2)
20 FORMAT(' ','X STEP=',F5.2, ' Y STEP=', F5.2)
*MAIN CODE
****************************************************************
*PROMPT FOR NAME OF FILE AND INPUT
PRINT *,'PLEASE INPUT A FILE NAME INCLUDING AN EXTENSION'
PRINT *,'ENTER QUIT OR Q TO EXIT'
READ (*,*) INNAME
*OPEN IF(0)
IF(INNAME.EQ.'Q'.OR.INNAME.EQ.'QUIT')THEN
PRINT *,'GOODBYE'
ELSE
*TEST FOR FILE
INQUIRE(FILE=INNAME, EXIST=EXISTS)
*IF EXIST=FALSE, DISPLAY MESSAGE
IF(.NOT.EXISTS) THEN
PRINT *,'FILE DOES NOT EXIST, PLEASE TRY AGAIN'
*ELSE EXIST=TRUE, OPEN (UNIT=1)
ELSE
OPEN(UNIT=1, FILE=INNAME, STATUS='OLD')
END IF
*************************************************************
*ASK CHOICES FOR OUTPUT
PRINT *,'PLEASE ENTER A NAME FOR AN OUTPUT FILE W/ EXTENSION'
PRINT *,'ENTER QUIT OR Q TO EXIT'
READ(*,*)OUTNAME
IF(OUTNAME.EQ.'Q'.OR.OUTNAME.EQ.'QUIT')THEN
PRINT *,'GOODBYE'
ELSE
*************************************************************
*CHECK IF DONE (IF 1)
IF(DONE.EQ.0)THEN
*OPEN THE OUTPUT FILE DO(2)
DO WHILE(USED.EQ.0)
INQUIRE(FILE=OUTNAME,EXIST=EXISTS)
*OPEN IF(2) FOR IF FILE ALREADY EXISTS
IF(EXISTS.EQ.1)THEN
OPEN(UNIT=2,FILE=OUTNAME,STATUS='OLD')
*END IF FOR IF FILE ALREADY EXISTS AMD OPEN IF DID NOT EXIST,
ELSE
OPEN(UNIT=2,FILE=OUTNAME,STATUS='NEW')
*END IF (2)
END IF
*READ IN 1 INTEGER (MAXIMUM VALUE OF 100) FOR # OF POINTS IN FILE
READ(1,*, END=111),POINTS
*READ IN X AND Y VALUES WITH MAX OF 100
DO I=1, POINTS
READ(1,*,END=111) X(I),Y(I)
END DO
*SORT POINTS
E = POINTS - 1
MORE = .TRUE.
DO WHILE (MORE .EQ. .TRUE.)
MORE = .FALSE.
DO I=1, E
IF(X(I).GT.X(I+1)) THEN
MORE = .TRUE.
A=X(I)
B=y(I)
X(I)=X(I+1)
Y(I)=Y(I+1)
X(I+1)=A
Y(I+1)=B
END IF
END DO
E=E-1
END DO
*FIND MIN AND MAX OF X AND Y
I=1
XMIN = X(I)
XMAX = X(POINTS)
YMIN = Y(I)
YMAX = Y(I)
DO I=1, POINTS
IF(Y(I) .LT. YMIN) THEN
YMIN = Y(I)
END IF
IF(Y(I) .GT. YMAX) THEN
YMAX = Y(I)
END IF
END DO
*fIND X AND Y STEPS
XSTEP = (XMAX-XMIN)/POINTS
YSTEP = (YMAX-YMIN)/POINTS
*PRINT TO THE OUTPUT FILE
DO I=1, POINTS
WRITE(2,5)I,X(I),Y(I)
END DO
WRITE(2,10)XMIN,XMAX
WRITE(2,15)YMIN,YMAX
WRITE(2,20)XSTEP,YSTEP
*PRINT GRAPH
*END DO (2)
END DO
*END IF (1)
END IF
*END IF QUIT(S)
END IF
END IF
*CLOSE THE FILES
CLOSE(UNIT=1)
CLOSE(UNIT=2)
111 PRINT * , 'The Program Has Now Ended'
*STOP
STOP
*END
END

Answer 1

if you want something quick and dirty, the easiest approach by far is to plot the graph sideways (not as bad as it sounds if you are going to print to paper). then you can just print each line with a length proportional to the y (now x) value (with a * at the end, or all *s for a histogram):
+-------------------------------------> y
| *
| *
| *
. etc
otherwise, if you don't want to use a library, things get complicated quite quickly. the next simplest approach is to make a 2d array of characters (the size of your screen), initially filled with spaces, and then "plot" to that; once done you can print it by reading the characters row by row and printing them. the advantage there is that it separates the axes from the data - if you try to do everything as you print a line, the different tasks become mixed up.
but plotting soon becomes more complex than you expected. for example, choosing tick marks and printing labels is itself a major task to do well. you're really better using a library (which is why i suggested the sideways approach first - if this is so quick + dirty than you don't want to use a library, then that is often good enough).
(this is the kind if thing you can do very quickly - i threw that together in a few minutes)
update: for tickmarks (it's getting too complicated!) you want the graphics gem algorithm - it's pretty much the standard, and the paper is easy to understand. that gives you your total range (as well as the tick spacing), and then scaling is easy (a character is "worth" (xmax-xmin)/nchars, as you say - not sure what more to explain?).