Line graph: reset previous point between dataxy series to prevent unwanted poly‑line connection

[katty0324]

What problem does this PR solve?

When a Gruff::Line graph is built with multiple dataxy series, the last
point of series N is drawn as a straight line to the first point of
series N + 1.
The result is a single, continuous poly‑line instead of visually separated
series, which is misleading for end‑users. :contentReference[oaicite:0]{index=0}

How to reproduce

require 'gruff'

g = Gruff::Line.new(600)
g.title = 'Gruff Line #dataxy connection bug demo'

# Series‑1: rising line
g.dataxy('Series 1', [0, 1, 2, 3], [1, 2, 3, 4])

# Series‑2: falling line – should start a new poly‑line,
# but is currently connected to the last point of Series‑1.
g.dataxy('Series 2', [0, 1, 2, 3], [3, 2, 1, 0])

g.write('before_fix.png')

Current output:
A “V”‑shaped line connecting (3,4) → (0,3) instead of two independent lines.
Expected output:
Two distinct polylines that do not share a segment.

Root cause

Inside Gruff::Line#draw_graph, the helper variables prev_x and prev_y
cache the coordinates of the previously plotted point.
They are initialised once before the outer loop that iterates over each
data_row (series).
Consequently, the starting point of the next series inherits the last point of
the previous series.

What does this PR change?

A single line resets the cache at the top of the loop:

 store.norm_data.each do |data_row|
+  prev_x = prev_y = nil
   data_row.coordinates.each_with_index do |(x_data, y_data), index|

By setting prev_x and prev_y to nil for every new series, the drawing
logic starts a fresh poly‑line and no longer links series together.

Impact / backwards compatibility

• Only affects Gruff::Line with the dataxy API.
• No public API changes.

Thanks for maintaining Gruff! Any feedback is welcome.

[Watson1978]

Thanks