Always provide a screenshot of what you are seeing and a Paint drawing of what you want to achieve
Change
sf::VertexArray lines = sf::VertexArray(sf::Lines, vertices.getVertexCount());
for(int iii = 0; iii < lines.getVertexCount()-1; iii+=2) {
lines[iii] = vertices[iii];
lines[iii+1] = vertices[iii+1];
lines[iii].color = sf::Color(255,(iii)*50,255,255);//sf::Color::White;
lines[iii+1].color = sf::Color(255,(iii+1)*50,255,255);//sf::Color::White;
}
to
sf::VertexArray lines = sf::VertexArray(sf::LinesStrip, vertices.getVertexCount());
for(int iii = 0; iii < lines.getVertexCount(); iii+=1) {
lines[iii] = vertices[iii];
lines[iii].color = sf::Color(255,(iii)*50,255,255);//sf::Color::White;
}
As the primitive type sf::Lines require 2x points than lines you draw. sf::LinesStrip should do the trick in VertexArray.