General Purpose Graph Plotter

General Purpose Graph Plotter is a bar-chart program that plots up to three data series simultaneously on a shared axis, each representing 12 months of numeric data. Up to 12 named files (labeled A–L) can be stored in a two-dimensional array g(12,12), with five-character labels held in the string array g$(12,5). The program uses a two-character instruction-code menu system (e.g., “e(a-l)” to enter data, “1-3(a-l)” to plot, “sv” to save, “fl” to view file values) and draws bars using PLOT/DRAW with INK color and OVER mode to distinguish the three plot positions. A sentinel value of −0.1 is used to mark uninitialized or “stop” data entries within a file.

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Program Analysis

Program Structure

The program is organized into clearly labeled sections accessed via GO TO and GO SUB. Execution begins at line 10 with a splash screen, then jumps to the Initialize Routine at line 1360 before entering the main menu loop at line 250. The major sections are:

1. Lines 10–60: Splash screen and startup
2. Lines 70–90: Subroutine to draw horizontal grid lines
3. Lines 110–240: Bar plotting routine
4. Lines 250–560: Main menu input loop and screen setup
5. Lines 570–800: Data entry (file input) routine
6. Lines 810–920: Plot dispatch routine
7. Lines 930–1270: File value display routine
8. Lines 1280–1340: Subroutine to print file list on main screen
9. Lines 1350–1350: Subroutine to draw screen border
10. Lines 1360–1450: Initialization routine
11. Lines 1460–1640: Instruction screen and final SAVE line

Data Storage

All data is held in two arrays declared at initialization:

• DIM g(12,12) — a 12×12 numeric array storing up to 12 data points (months) for each of 12 named files (A–L).
• DIM g$(12,5) — a 12-element string array holding a five-character label for each file.

Files are addressed by converting the letter code (a–l) to an array index via CODE a$(2)-96, exploiting the ASCII layout where CODE "a" = 97, yielding index 1 through 12. The sentinel value s=-.1 (line 1450) marks uninitialized or terminating data entries, allowing partial-month datasets.

Menu System

The menu at line 250 uses a two-character string a$ entered via INPUT. Commands are dispatched through a cascade of IF tests. The full command set is:

Code	Action
st	STOP the program
in	Show instruction screen
cl	Clear plots (new screen setup)
e(a-l)	Enter/update a data file
1-3(a-l)	Plot file in position 1, 2, or 3
sv	SAVE program with data, auto-running at line 1460
vr	VERIFY tape save
fl	Display file values
co	COPY screen to printer
mx	Enter new maximum Y-axis value
nt	Enter a new chart title

Bar Plotting Routine (Lines 110–240)

The plotting routine draws three-pixel-wide bars for each of the 12 months. For each month n, the X position is n*16 and the bar height is scaled as z*(100/m), where m is the current Y-axis maximum. The three plot positions p1, p2, p3 (values 44, 49, 53) provide a small horizontal offset so bars for different series sit side by side.

INK colors differentiate the series: INK 1 for position 1, INK 2 for position 2, INK 8 (black) for position 3. The middle pixel of each bar is drawn with OVER 1 (line 220), which XORs it onto the display — this is used to provide a visual highlight stripe down the center of each bar. A bar that would exceed the maximum is clamped to m with a flag za=1 that adds one extra pixel to indicate clipping.

Negative or zero values are handled at line 230: if z<=0, a single pixel is plotted at the baseline (y=49) using OVER o.

Month Label Strings

Month abbreviations are stored across three string variables that together spell out the months vertically:

• l$ = " J F M A M J J A S O N D"
• m$ = " a e a p a u u g e c o e "
• n$ = " n b r l y n l t p t v c"

These are printed as three rows on the graph screen (line 530) and also used in the file values display (lines 1060–1090), where individual characters are accessed by index v=d*2 to step through the two-character-spaced string.

File Values Display (Lines 930–1270)

The file viewer at line 930 uses a column counter c starting at 5 to position up to three file columns on screen, incrementing by 9 each time a file is selected (line 1190). A guard at line 1120 wraps back to the start if c>=29. Files A–L are listed using CHR$ arithmetic to print the letter labels alongside their stored names from g$.

ON ERR and RESET Usage

Line 280 opens with ON ERR RESET before the INPUT statement. This ensures that if a BASIC error occurs during input (such as a type mismatch), the system resets the error handler rather than crashing, providing a degree of robustness to the menu loop.

Bugs and Anomalies

• Line 400 — erroneous AT syntax: PRINT AT 1,8,, AT 1,8;h$ contains a double comma which is syntactically odd; the intent is to clear then reprint the title, but the double comma is likely a typo for a separate statement or a semicolon.
• Line 900 — incorrect offset: LET t=(CODE a$-48) operates on the full string a$ rather than a$(1), so CODE a$ returns the code of the first character — which for “1”, “2”, “3” gives 49, 50, 51, yielding t values of 1, 2, 3. This happens to work correctly but is not explicit in its intent.
• Line 1200 — misplaced PLOT: PLOT 255,0 plots to the bottom-right corner of the screen, which appears to be a stray or accidental statement unrelated to the displayed data.
• Line 1640 — unreachable SAVE: CLEAR :SAVE "GPGP" LINE 10 at line 1640 is never reached during normal execution (no GO TO 1640 exists); it appears to be a developer convenience line to re-save the program from the editor.
• Line 970–990 — label arithmetic off-by-one risk: The file list display in the fl routine uses CHR$(48+e), CHR$(52+e), CHR$(56+e) for e from 17 to 20, producing characters at codes 65–68, 69–72, 73–76 — the uppercase letters A–L spread across three columns. This is clever but fragile if the loop range changes.

POKE Usage

Lines 20 and 260 both execute POKE 23609,15, which sets the border color attribute byte (system variable BORDCR) to 15 — white paper, black ink — ensuring consistent border rendering after screen operations that might alter it.

Line 950 uses POKE 23693,7, which sets the system variable ATTR_T (temporary color attributes) to value 7 (white paper, black ink, no flash/bright) before the file values display.