Min Max typo bug in xyseries.c

[jakelewis3d]

void slope_xyseries_update(SlopeXySeries *self)
{
SlopeXySeriesPrivate *priv = SLOPE_XYSERIES_GET_PRIVATE(self);
SlopeScale * scale = slope_item_get_scale(SLOPE_ITEM(self));
const double * x = priv->x_vec;
const double * y = priv->y_vec;
priv->x_min = priv->x_max = x[0];
priv->y_min = priv->y_max = x[0]; //was
priv->y_min = priv->y_max = y[0]; //should be
long k;