Make grid configurable

[federico-razzoli]

After #54 , grid is a boolean setting. Let's preserve this for simplicity. But optionally, one should be able to set it to a dictionary of grid characteristics, supporting the whole Gnuplot syntax:

# Basic grid control
set grid                    # Enable grid
unset grid                  # Disable grid

# Grid line styles
set grid linetype 0         # Use default linetype
set grid linetype 1         # Use linetype 1 (solid)
set grid linetype 2         # Use linetype 2 (dashed)
set grid linetype 3         # Use linetype 3 (dotted)

set grid linewidth 0.5      # Thin grid lines
set grid linewidth 1        # Normal grid lines  
set grid linewidth 2        # Thick grid lines

set grid linecolor rgb "gray"     # Gray grid
set grid linecolor rgb "#cccccc"  # Light gray grid
set grid linecolor rgb "black"    # Black grid

# Combined style settings
set grid linetype 2 linewidth 0.5 linecolor rgb "gray"

# Control which axes show grid
set grid xtics              # Grid for x-axis tics only
set grid ytics              # Grid for y-axis tics only  
set grid xtics ytics        # Grid for both axes (default)
set grid noxtics            # No x-axis grid
set grid noytics            # No y-axis grid

# Control major vs minor grid lines
set grid xtics mxtics       # Major and minor x-axis grid
set grid ytics mytics       # Major and minor y-axis grid

# Layering (where grid appears)
set grid back               # Grid behind data (default)
set grid front              # Grid in front of data