Graph functions in your terminal.

lgraph

#!/usr/local/bin/lua
usage_str = [[
Usage:
  lgraph 'function of x'

Example:
  lgraph 'x^2'
]]

Grapher = {xmin = -1, xmax = 1, ymin = -1, ymax = 1, ncols = 160, nrows = 50}

function Grapher:new ()
  return setmetatable({}, {__index = self})
end

local function round(x)
  return math.floor(x + 0.5)
end

-- This returns a *function* that maps [a1, b1] to [a2, b2].
local function range_mapper(a1, b1, a2, b2)
  return function (x)
    local perc_from_a = (x - a1) / (b1 - a1)
    return a2 + (b2 - a2) * perc_from_a
  end
end

-- Makes a map self.char[col][row] = <nil or character to print>.
function Grapher:setup_char_table(eqn)
  local f = loadstring('local x = ...; return ' .. eqn)

  -- This will be a table of tables so that self.char[col][row] is either
  -- nil or the character to print at that location.
  self.char = {}

  local y_to_row = range_mapper(self.ymin, self.ymax, self.nrows, 1)
  local col_to_x = range_mapper(1, self.ncols, self.xmin, self.xmax)

  for col = 1, self.ncols do
    local y = f(col_to_x(col))
    local row = round(y_to_row(y))

    if self.char[col] == nil then self.char[col] = {} end
    self.char[col][row] = 'o'
  end
end

function Grapher:graph(eqn)
  self:setup_char_table(eqn)

  for row = 1, self.nrows do
    for col = 1, self.ncols do
      io.write(self.char[col][row] or ' ')
    end
    io.write('\n')
  end
end

AxisGrapher = Grapher:new()

function AxisGrapher:graph(eqn)
  self:setup_char_table(eqn)  -- This part is the same.

  local zero_col = round(range_mapper(self.xmin, self.xmax, 1, self.ncols)(0))
  local zero_row = round(range_mapper(self.ymin, self.ymax, self.nrows, 1)(0))

  for row = 1, self.nrows do
    for col = 1, self.ncols do
      local c = self.char[col][row]
      if not c and row == zero_row then c = '-' end
      if not c and col == zero_col then c = '|' end
      io.write(c or ' ')
    end
    io.write('\n')
  end
end

if not arg[1] or (arg[1] == '-a' and not arg[2]) then
  print(usage_str)
  os.exit(0)
end

if arg[1] == '-a' then
  g = AxisGrapher:new()
  arg[1] = arg[2]
else
  g = Grapher:new()
end

g:graph(arg[1])

I made this for a Lua tutorial video I'm working on. Its purpose is to illustrate how to handle simple object-oriented patterns in Lua.

Oh, you can also graph things with it. Here's an example:

$ lgraph 'math.sin(7 * x)'

            ooo                                oooo                             
          oo   oo                             o    o                            
         o       o                           o      o                           
        o                                   o        o                         o
                  o                        o          o                         
       o           o                      o                                   o 
      o                                                o                     o  
                    o                    o              o                       
     o               o                                                      o   
                                        o                o                 o    
    o                 o                o                                        
   o                                                      o               o     
                       o              o                    o                    
  o                     o                                                o      
 o                                   o                      o           o       
                         o          o                        o                  
o                         o        o                                   o        
                           o      o                           o       o         
                            o    o                             oo   oo          
                             oooo                                ooo         

I modified the ncols and nrows values so the above example will make sense when viewed on this gist page.